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Education Notes from the Lab

Beyond Colour: The Rheology and Viscosity of Acrylic Paints

In the post-modern era of social media sharing and digital art, the physical, material properties of paints often take a back seat to the immediacy of their colour. Today’s acrylic paint comes formulated in a range of mediums from free flowing, ink-like forms to highly bodied, gel types. Each has their purposes, that can either aid, or work against the artist in bringing to life their vision. Rheology and viscosity are the key properties that form the real structure and working properties of acrylic while pigment is essentially decorative, a metaphoric coat of paint over the architectural acrylic armature.

While perhaps too technical sounding for some painters, learning these concepts will pay dividends to those who wish to truly understand their medium and how to build a painted surface. In addition to creating new work, these concepts offer a window of understanding to the works of the past – both to the grand tradition of painting in oil, and to the modern experimentations of 20th century artists. Today’s contemporary formulations of acrylic paint have come a long way to find the best from both of these domains, encompassed by Tri-Art’s philosophy for artist’s finest quality acrylic paint.

The Basics: The Long and Short of Rheology

Viscosity is the measure of a fluid’s resistance to flow under applied force. It’s an intuitive measure of simply how thick or thin a liquid is. Viscosity, however, is the greyscale of paint properties in a colourful world of other paint behaviours. Rheology is one of the true superstars. Analytically, it describes a fluid’s deformation and flow under applied stress.

Paints with long rheology are flowing and stringy, like honey and glue. The force of gravity alone is enough to make long rheology paints flow after they dry, resulting in softened edges and smooth surfaces devoid of brush marks. This is often utilized in self-leveling formulas for wall paint, primers, or artist’s gesso. Traditional painting techniques like Indian Rogan for textiles also utilize this stringy rheology to great effect in complex designs.

Fluids with short rheology, in contrast, are unable stretch or flow. They are often better described as ‘soft solids’ – meaning the physical action of a brush or palette knife results less in flow, and more in the paint being deformed around the artist’s tool, recording the act of mark making.[i] When a tool is lifted from the surface of these paints, the paint reaches a quick breakpoint, resulting in stiff peaks and veining. This behaviour was long the coveted purview of oil paint.

The Writing is in the Rheology: Reading Historical Brushwork in Oil Painting

Since its advent, painters have been exploring the rheological properties of oil paints. In its purest form, high-quality oil paints have a very short rheology. It preserves an incredibly high-fidelity record of an artist’s marks on a canvas. Think of Jean-Paul Riopelle’s sculptural taches, Rembrandt’s thickly piled white highlights, and Van Gogh iconic brushwork. The three-dimensional quality of a painting truly brings the object to life. Perhaps the most obvious case for this is when encountering modern canvas prints, or even studio copies of famous paintings – their flatness can land them somewhere in the uncanny valley, if not immediately signifying their less-than status.

Van Gogh's 1889 Sunflowers and detail showing the thick brush strokes of paint.

Vincent Van Gogh. Sunflowers. Oil on Canvas. August 1889. Van Gogh Museum, Amsterdam.  Right, Detail of heavily textured oil paint.
Rembrandt van Rijn Portrait of a Man with Arms Akimbo, an example of thickly piled lead white paint. Rembrandt van Rijn. Portrait of a Man with Arms Akimbo. Oil on Canvas. 1658. Agnes Etherington Art Gallery, Kingston, ontario. A local treasure and example of how lead white paint was used to create textured paint surfaces.

Unlike acrylic, the rheology of oil paint is heavily, if not exclusively determined by the pigment choice and its concentration in the paint. Lead white is probably the most famous colour for its unique properties in oil paint. The resulting paint is high viscous but also thixotropic. Another rheological superstar term, thixotropy is often used to describe a thick paint that becomes more fluid with agitation, but quickly regains any of its gel-like structure when at rest again. Although this too may sound esoteric, the thixotropic quality of lead white paint was deeply coveted – it gave artists a thickly bodied paint that need not be thinned by oil or solvent to make fluid, instead it responded effortlessly to the touch of a brush, while retaining form and brushwork.

Throughout the 18th and 19th century thixotropic paint mediums were also highly prized. Under the influence of experimental painters like Joshua Reynolds and J.M.W. Turner, thixotropic mixtures of lead dryers, oil and resin called megilps were popular. [ii] These gelled mediums gave an immediately bodied texture, flow and transparency to brushwork in oil paints. They found extensive use in creating thickly bodied glaze layers that previous could only have been thin veils of colour.

Dort or Dordrecht: The Dort packet-boat from Rotterdam becalmed

JMW Turner. Dort or Dordrecht: The Dort packet-boat from Rotterdam Becalmed. Oil on Canvas. 1818. Yale Center for British Art. Artists like JMW Turner and Joshua Reynolds are well known to have modified their oil paint rheology with mediums like megilps.

Oil paints unfortunately have a complex chemistry of drying, aging and degradation that is readily disrupted by the inclusion of additional media in the quest for modified rheology. The consequence of megilps were quickly apparent with many works deteriorating within years of their completion. Paint layers darkened, cracked and fell off canvases. Adding further insult to injury, heat treatments intended stabilize flaking paint squashed any traces of the brushwork the artist intended to build. The effects were so disastrous for artists like Reynolds that an exhibition of his deteriorated works was staged with the sole goal of dissuading contemporary artists from further using megilps and the likes of.[iii] By the end of the 19th century, however, painting would take on a new direction that meant a fall from popularity for these problematic mediums.

“…cold and often without feeling.” The History of Early Acrylic Paints

After the late 19th century Impressionists further explored the representation of reality through light, colour and texture, artists of the 20th century began exploring more overt abstraction, colour, composition, and flatness in painting.

In the 1940s, painters like Morris Louis were looking for colourful and liquid paint to create their design focused composition and abstract works. Experimental artists like Picasso had pioneered the use of commercial house paints, like Ripolin, in the previous decade. Commercial house paints offered an economical, pre-formulated source for liquid paint, but with several caveats – they offered a limited range of colour and were greatly lacking quality when compared to artist’s oils. Commercial house paints are made with a mixture of opaque pigments and fillers engineered to give coverage and mass tone as the paint is supplied. The pigments are often poorly dispersed and historically of less than archival quality. The resulting paint would be difficult to mix with, creating a tendency toward a muddy palette. This perhaps informed Picasso’s own colour choices for much of his Cubist period.

Picasso's girl with a mandolin, an example of early use of house paint in fine art painting

Pablo Picasso, Girl with a Mandolin (Fanny Tellier). Oil on canvas. 1910. Museum of Modern Art, New York. Picasso is credited as first fine artist to adopt fluid, flat-dryng house paints

In attempts to make their own colourful, liquid paint, artists like Louis would heavily thin artist’s oil paints. The results were mixed – the excessive amount of solvent necessary to liquify artist’s oils meant the resulting solution was weak in pigment and prone to separation. In works by Helen Frankenthaler, we can see the deleterious effects of overly thinned oil poured onto raw, unprimed canvas: a halo of discoloured oil has separated from the paint where it was applied. Additionally, oil is well known to accelerate the deterioration of canvas due to high acid content, making these works likely unstable for the long term.[iv]

The first acrylic paint for artists was developed in the late 1940s, in part to address the lack of fluid alternatives to artist’s oil paint. Magna® was invented by Leonard Bocour and Samuel Golden in conjunction with Rohm and Haas, the principle manufacturer of acrylic resins then, and to this day. Leonard Boccour “wanted something with viscosity, something that could simulate oil and they thought in terms of house paint, something that was very loose and liquidy and very, very flat.” Magna® was advertised as a fast drying, flexible alternative to oils. Its very high pigment concentration allowed the paint to be thinned while keep intense saturation for staining and pouring. Magna® was hugely influential on the artists of North America and later the UK. For Louis, Magna® was critical for achieving his thin veils of colour and intense stains. These effects could not be produced by traditional oil paint or wall paint. [v]

magna solvent acrylic paints

Magna® Solvent Acrylic paints. Image by Seventex via Wikipedia. CC 3.0. Magna® was the first artists’ solvent acrylic paint available.

Despite its breakthrough technology, Magna® was not without problems. The paint required users to work with large amounts of solvent (Louis was known to use four gallons for a painting), the paint could be resolubilized and disturbed by the application of further layers, and over thinning of the paint resulted in cracking and separation on the canvas. [vi]

Henry Levinson, founder of Permanent Pigments was one of the first to see the advantages of using water-based acrylic emulsion for artist’s paints – it could be thinned with just water, and could be layered with the same paint once it was dry. In 1955 he developed one of the first artist’s acrylic emulsion paints, Liquitex®. Other manufacturers quickly followed suit. Accounts of these early acrylic paints note it was thin, runny, and not generally commercially successful. Frankenthaler described it as “often very cold and often without feeling.” However, for many painters of the mid-20th century, the rheology of this paint became intrinsically linked to their expression as an artist. Modern painters felt that a flat surface, devoid of brush marks was impersonal, but also critical to their practices. A lack of texture highlighted their contemporary use of colour and composition – rather than referencing the grand tradition of oil painting, optical effects, and representation.  Andy Warhol felt very strongly about this surface quality, seeking uniformity, devoid of artistic intention in his repetitious pop-art works like the now famous Marilyn Diptych (made with layers of oil-based silkscreen and hand paint acrylic emulsion paint). A quote from Patrick Caufield summarizes the feelings of the era, “I’m not asking for brushstrokes. I haven’t got any brushstrokes, you know; I’m not Rembrandt.” [vii]

Instead of thixotropy, the terminology of these paints revolves around utilitarian properties like sag resistance.[viii] Wall paint is designed to become fluid as it is stirred and shaken, but gel upon rest, creating drip-free formulations for brushing and rolling vertical surfaces. This was balanced with self-leveling properties to given flat, even coatings – those with long rheology that are able to slightly flow under the force of gravity alone, but not completely flow off the wall.[ix] The same technology was used to formulate the earliest artists’ acrylic paint, resulting in similar properties – flat, even surfaces.

Acrylic paint with oil-like viscosity and gel mediums were eventually introduced in the 1960s by several manufactures, but were of similar quality to earlier liquid acrylics. Despite being thick, these paints had long, stringy rheology that slumped and flowed as they dried. Additionally, painters still trying to heavily thin their paints, now with water, still encountered the same issues as overly thinned Magna®, with cracking and pigment separation. [x] Acrylic as we know it today was still in its infancy.

Contemporary Chemistry: Formulating Modern Acrylic Rheology

Water-based acrylics have come a long way since their advent in the 1950s. Once dismissed as too thin or glue-like paint, acrylic is now versatile enough to replace nearly every other artist’s medium. Tri-Art considers acrylic paint to be the great imitator of everything – it has an amazing capacity to replace artists’ materials historically made from a range of oils, resin and solvents. It is largely by modifying acrylic’s viscosity and rheology that this medium is able to shape shift into such a wide range of artists materials. Manufacturers like Tri-Art produce not only acrylic paints, but also a range of inks, screen printing media and pouring fluids.

inks on a brush and a nibimage of acrylic based screen printing ink being poured onto printing screen

Acrylic is the great imitator of all things. More than just paint,  Tri-Art produces inks and screen printing mediums. Creating the proper viscosity and rheology allows acrylic to morph into this spectrum of possible materials.

Because of their watery, primal state, all acrylic products are created with the aid of rheological modifiers. The three main types are celluloses, polyacrylates, and associative thickeners. Cellulose thickeners are the oldest thickeners available. These are chemically modified cellulose polymers that gel the water component of paint to give it body, but also thin with mixing and brushing. This property, called shear-thinning, gives paint good brushing qualities and cuts down on splattering while rolling. Cellulose thickened paints have long, stringy rheology that result in self-leveling properties that are often exploited in wall paints and gessoes. Polyacrylates thickeners, in contrast, form firm gels with short rheology. Many of us will be familiar with these gelling agents as Carbopol, the agent used to give body to hand sanitizer and many other personal care products. The last category, associative thickeners, consist of a wide range of small molecules that create bridges between the paint components. Rather than greatly thicken the acrylic paint, associative thickeners can be utilized to stabilize the pigment content, while still allowing for flow and leveling properties. Often these thickeners are thixotropic in nature, keeping pigment suspended in a thickened matrix, but readily thinning out upon shaking and brushing.[xi] By selecting amongst these rheological agents and other components, modern acrylic paint and mediums can be engineered to encompass a wide range of properties that solve the issues faced by artists of the past.

Fine Art Philosophy for the Mass Market Medium: Tri-Art Acrylics

At Tri-Art, our acrylics were developed with a deep appreciation for the hand of the artist. When developing Tri-Art’s acrylic, it was imperative that the paint had excellent handling, with a short rheology that could match that of oil paint. Unlike existing acrylics, Tri-Art didn’t want to develop an artist paint using the principles of house paint formulation. Tri-Art wanted to make something that was like oil paint, but with endless possibilities of acrylic. Many other acrylic brands still utilize thickeners that result in paint that is heavily bodied and viscous, but also stringy and glue-like paint that loses resolution as it dries. With Tri-Art acrylics, the short rheology yields a paint that effortlessly moves but also holds a crisp record of your brushwork after drying. This includes not only our Finest Quality High Viscosity line, but virtually all our acrylic paints.

sharp peaks of high viscosity red acrylic paint illustrating the very short rheology

The sharp, high-fidelity peaks of Tri-Art’s Finest High Viscosity acrylic paint hold a record of your mark making, even after drying. Our commitment to quality means that you can sculpt tall peaks of paint onto the canvas without worrying it will fall off.

Unlike the disastrous experiments medium use in oil painting, or the early experiments of overly thinned acrylics, todays contemporary acrylic paints offer artist the ability to explore further with confidence. Beyond mere colour, understanding the rheology of acrylic paint can truly help you to achieve the paint surfaces you envision. For those who wish to paint with body, Tri-Art offers high viscosity paints and mediums. For those who wish to paint with fluidity, Tri-Art offers low viscosity paints, along with liquid glass pouring mediums, self-leveling gels and more. With a selection of these paints and mediums you can create, or recreate, a nearly infinite range of surface textures and effects.

For a masterclass in acrylic painting and mediums consider reading Rheni Tauchid’s Acrylic Painting Mediums and Methods[xii], or New Acrylics Essential Sourcebook[xiii].

gel medium in jar.

Notes:

[i] Daniel Blair, ‘Viscoelastic Properties: The Rheology of Soft Solids’, in Molecular Gels: Structure and Dynamics (Cambridge: Royal Society of Chemistry, 2018), pp. 28–56 <https://doi.org/10.1039/9781788013147-00028>.

[ii] Leslie Carlyle, ‘Building Visual Evidence of Past Practices in the Creation of Oil Paintings’, in A Changing Art: Nineteenth-Century Painting Practice and Conservation, ed. by Nicola Costaras and others (London: Archetype Publications Ltd., London, United Kingdom, 2017), p. pp.23-36, 11 figs. (9 color), 8 notes, refs.

[iii] Carlyle.

[iv] Jo. Crook and Tom. Learner, ‘The Impact of Modern Paints’ (New York: Watson-Guptill, 2000), p. 192 p. <file://catalog.hathitrust.org/Record/004084627>.

[v] Crook and Learner.

[vi] Crook and Learner.

[vii] Crook and Learner.

[viii] ‘Rheology and Interfacial Measurements for Coatings, Paints and Inks’, Center for Industrial Rheology <https://www.rheologylab.com/articles/coatings-paints-inks/> [accessed 8 March 2021].

[ix] Adrian Hill, ‘Rheology for Coatings’, Paint and Coatings Industry, 22.3 (2006), 52–57.

[x] Crook and Learner.

[xi] ‘Rheology Modifiers Selection for Paints & Coatings’, SpecialChem <https://coatings.specialchem.com/selection-guide/rheology-modifiers-selection-for-waterborne-and-solventborne-coatings> [accessed 9 March 2021].

[xii] Rheni Tauchid, Acrylic Painting Mediums & Methods : A Contemporary Guide to Materials, Techniques, and Applications Acrylic Painting Mediums and Methods, First edit (New York, New York: Monacelli Studio, 2018).

[xiii] Rheni Tauchid, New Acrylics Essential Sourcebook : Materials, Techniques, and Contemporary Applications for Today’s Artist  (New York: Watson-Guptill Publications, 2009).

Bibliography

Blair, Daniel, ‘Viscoelastic Properties: The Rheology of Soft Solids’, in Molecular Gels: Structure and Dynamics (Cambridge: Royal Society of Chemistry, 2018), pp. 28–56 <https://doi.org/10.1039/9781788013147-00028>

Carlyle, Leslie, ‘Building Visual Evidence of Past Practices in the Creation of Oil Paintings’, in A Changing Art: Nineteenth-Century Painting Practice and Conservation, ed. by Nicola Costaras, Kate Lowry, Helen Glanville, Pippa Balch, Victoria Sutcliffe, and Polly Saltmarsh (London: Archetype Publications Ltd., London, United Kingdom, 2017), p. pp.23-36, 11 figs. (9 color), 8 notes, refs.

Crook, Jo., and Tom. Learner, ‘The Impact of Modern Paints’ (New York: Watson-Guptill, 2000), p. 192 p. <file://catalog.hathitrust.org/Record/004084627>

Hill, Adrian, ‘Rheology for Coatings’, Paint and Coatings Industry, 22.3 (2006), 52–57

‘Rheology and Interfacial Measurements for Coatings, Paints and Inks’, Center for Industrial Rheology <https://www.rheologylab.com/articles/coatings-paints-inks/> [accessed 8 March 2021]

‘Rheology Modifiers Selection for Paints & Coatings’, SpecialChem <https://coatings.specialchem.com/selection-guide/rheology-modifiers-selection-for-waterborne-and-solventborne-coatings> [accessed 9 March 2021]

Tauchid, Rheni, Acrylic Painting Mediums & Methods: A Contemporary Guide to Materials, Techniques, and Applications Acrylic Painting Mediums and Methods, First edit (New York, New York: Monacelli Studio, 2018)

———, New Acrylics Essential Sourcebook: Materials, Techniques, and Contemporary Applications for Today’s Artist  (New York: Watson-Guptill Publications, 2009)

 

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Education Notes from the Lab

Mulling it Over: Contemporary Pigment Grinding

The founder and owner of Tri-Art, Steve Ginsberg, often reminds us that paint is the raw material of an artwork – an artist must endeavour to transform paint into a finished piece. Just as paint by itself does not make a painting, pigments do not simply make paint – the manufacture of paint requires the careful manipulation of raw ingredients and techniques to produce not only paint, but high-quality artists’ paint. Pigment grinding is the essential step that allows powdered pigments to be transformed into smooth, liquid paint. The process is in itself an art form that receives little attention, even in the world of artists’ supplies. While pigment load is often given primacy over all else when evaluating paint, the quality of the pigment’s dispersion is just as critical when bringing out the best in each pigment.

Aggregating Data: A Brief Primer on Grinding Pigments

While pigments appear to be fine powders composed of evenly ground particles, these particles clump together on a very small-scale forming aggregates. These clumped together particles can have significant effects on the final appearance of the paint: they can appear as physical gritty particles, lower the colour intensity (chroma) of the paint, and effect the opacity or transparency of the paint. Clumped particles can lower the opacity of an opaque colour, or confusingly lower the transparency of a transparent colour. When aiming for opacity, dispersing these particles is critical to attainting maximum covering powering. When it comes to transparent colours, these aggregated particles of pigment can also trap pockets of internal air that scatter light.[1] Rather than allowing light to pass through, these air pockets create paint that looks hazy, like frosted glass. Therefore, one of the primary functions of pigment grinding, as suggested by the name, is to break apart these aggregates to create an even dispersion.

Paint outs of Pyrrole Red acrylic Figure 1: Pyrrole Red. Left, poorly dispersed pigment shows a lack of transparency. Left, the well ground pigment has an excellent transparency and full depth of colour.

The traditional techniques of grinding pigment have gone unchanged for centuries. Handmade paint using a muller and glass slab is making a resurgence today. With gentle circles of the muller, the pigment is dispersed into a medium, like gum Arabic for watercolour or linseed oil for oil paints. The action of the muller against the glass shears apart the aggregated pigment particles and replaces any air with the binder of choice.

a glass muller with red pigment and a palette knife. An etching from 1751 showing a grinding stone muller with pigments.

Figure 2: Left, a modern glass muller. Right, a detail from the 1751 Encyclopedia of Diderot & d’Alembert showing the same. “Plate VI: Painting, Folding Easel and Traditional Easel, Pastel Box and Grinding Stone”. Image via The Encyclopedia of Diderot & d’Alembert Collaborative Translation Project. Ann Arbor: Michigan Publishing, University of Michigan Library, 2010. Web. [January 19, 2021]. <http://hdl.handle.net/2027/spo.did2222.

Acrylics: Modern and Mullers Don’t Mix

Despite a Renaissance of handmade paint, nearly all artists today buy their paint pre-made, and for good reasons. Working with dry, powdered pigments alone is health hazard, and quite a messy undertaking. When it comes to making acrylics, the process is even more difficult. Ignoring the complex formulation and stepwise process necessary to create acrylic paint from its raw materials, if you wanted to mix a pre-made acrylic medium with pigment you would find the results might not be what you hoped.  Acrylic emulsion paints cannot truly be mulled by hand like oil or watercolour because they rapidly dry and are not resoluble. Mixing a paint from pigment takes considerable time, the paint must be carefully mulled, tested, and mulled further, often with adjusting the amount of pigment or binder added. This is time that acrylic paint does not allow for.

Acrylic paints are really only possible thanks to the industrial and chemical advances of the 19th century and the post-war era. By the 19th century, oil paints were not being made just by hand with mullers. Mechanical, large-scale, roller mills were developed for commercial artists paint production. These allowed for larger volumes of paint to be produced, with more evenly dispersed pigments than could be produced by hand. With the invention of the metal paint tube in 1841, most artists would buy their paints pre-made from colourmen and art stores.[2] When water-based acrylics were introduced in the 20th century (in the early 1960s several companies introduced emulsion paints) they still, however, came with significant manufacturing challenges.

To overcome the challenges of grinding dry pigment directly into acrylic paint colourant dispersions (or pastes/slurries) became common in the 1970s.[3] These are highly concentrated, water-based precursors that allow a thorough grinding of the pigment without having to contend with the many other ingredients of acrylic paint. Many of us will be familiar with a similar product and concept, the liquid colourants added to an uncoloured can of commercial wall paint at a hardware store. These products however cannot hold a candle to artists quality paints that have been devotedly crafted.

Tri-Colour, Tri-Phase: Making Modern Pigment Dispersions

To produce colourful dispersions for acrylic paints, pigments are ground in three phases: wetting, dispersing and stabilizing. [4]  Most pigments are hydrophobic, meaning they do not mix with water, and would rather stick to themselves. Oil paints are at an advantage here, with many pigments readily absorbing oil and wetting out. Water-based dispersions thus require synthetic wetting agents. Although these may sound unfamiliar, wetting agents are nothing more than surfactants – the chemicals like sodium lauryl sulfate that make up our everyday detergents and soaps. They are molecules with polar and non-polar portions that bridge non-polar substances, like pigments, with highly polar water.

pigment float on top of water, and pigment dispersing into water.

Figure 3: Quinacridone red, without and with a dispersion agent (surfactant).

Tri-Art uses not one, but an array of surfactants that have come from years of testing. By carefully attuning the surfactant used, the maximum amount of pigment can be wet out and dispersed into the solution. By aiming for the highest concentration of pigment possible in these dispersions, Tri-Art has been able to create an incredibly high pigment load in their finest quality acrylic paints.

Once the pigment is wetted into water it is then ready for dispersion – the breaking of the aggregate particles. During this phase high mechanical forces are critical to grind the pigment. [5] At Tri-Art two processes are used to disperse the pigment as effectively and efficiently as possible. The first process is high speed mixing with a cowles blade. The outer edge of the blade reaches a speed magnitudes higher than the shaft of the mixer, thereby imparting high shear forces throughout the solution that break apart aggregate particles. This is typically able to produces particles smaller than 250 um, and comfortably within the range of 30-40 um that most artists pigments are formulated for use at.[6] For synthetic organic pigments like Quinacridone red that tend to stick together, a second step is necessary to further break up the very fine particles. Bead milling is able to decrease the particle size to less than ten um if desired. [7] This technique employees a cylinder filled with several kilograms of small industrial ceramic beads (less than two millimeters in diameter). The beads are spun by an agitator that cause the beads to collide with each other. Their resulting impact force grinds apart pigment aggregates between the ceramic balls. The process is energetic enough to produce a considerable amount of heat, so a constant flow of water in employed in a cold-water jacket to keep the dispersion from overheating.

pigment grinding article images 2021

Figure 4: No mullers will be found in modern paint making, instead a cowles blade and industrial ceramic beads (with pen for scale) are employed. Each bead is less than two mm in diameter.

pigment is mixed with an electric whisk into water. The mixture becomes a dark, saturated red, with a great deal of foam ontop.

Figure 5: Demonstrating dispersion by shear forces. A cowles blade is mimicked with a milk frother. This also emphasizes the need for another key additive in pigment dispersions: defoamers.

The last step of grinding the pigment into a concentrated solution is to stabilize the newly dispersed particles. Because of their affinity for each other, the particles must often be stabilized to prevent them from reforming the aggregate particles they once made up. Stabilizers are added that absorb onto the surface of each pigment particle, coating them in a repellent that balances all the particles an atomic distance apart from each other. These are most commonly acrylic polymer salts – chemically very similar to the acrylic polymers that will make up the finished paint, they will provide stabilization without interfering with the final formulation. The mixture can be further stabilized with acrylic resins (acrylic paint binder) forming a slurry that is then ready to be mixed into a multitude of paint formulations. [8]

Discerning Dispersions: Why Does the Quality of the Grind Matter?

Pigment grinding is critical to the final outcome of the paint because a high-quality grind allows for high pigment loading, but just as importantly, a high-quality grind allows for control of the pigment’s characteristic hue, transparency/opacity, gloss/matte qualities, and staining power. When it comes to single pigment artists’ qualities paints, it takes care to achieve not only an excellent quality pigment dispersion each batch, but one that is consistent from batch to batch too. For these single pigment paints, as the name suggests, any discrepancy in hue cannot be adjusted for with other colours – this would affect the mass- and undertones and colour mixing properties of the paint, critical for the painter who has selected these colours to work with.

Take for instance earth pigments like raw umber. It’s come be known as notoriously difficult pigment here at Tri-Art. The earth pigment contains a portion of inorganic iron oxide and another of organic carbon black. Inorganic pigments can typically be dispersed at high speed with a cowles blade in little time (20-40 minutes), but the organic carbon black remains as undispersed particles. The resulting colour is warmer than should be, with considerable streaks. Some manufactures might simply stop here with a ‘time is money’ attitude, but at Tri-Art we persevere on until all portions of the pigment are full dispersed – a product of careful surfactant attenuation and further grinding processes.

A draw down card of two raw umber samples and a macro detail shot of raw umber paint with pigment particles visible.

Figure 6: Far Left, draw down of raw umbers. Left, standard well dispersed dispersion. Right, carbon black has not been dispersed. Far Right, black particles of undispersed carbon black in raw umber paint.

In the lab we carefully test for the consistency of these colours with several methods: measuring the size of the particles with a fineness of grind gauge, making a small bespoke batch of paint to compare to our standards, and undertaking rub-up tests. The latter is particularly good for quick testing dispersions. A dispersion is coated onto a card and a small spot is rubbed (swirled) while still wet. After allowing it to dry, poorly dispersed, unstable particles will be shown by a colour change where the rub-up has disturbed them. In properly dispersed and stabilized mixtures the colour will remain largely the same, as the rub-up simply moves the homogenized, stable dispersion around. [9]

two paint draw downs with rub-up tests at their centre.

Figure 7: Left, a successful rub-up test. Right, an unsuccessful rub up test showing the disruption of poorly dispersed pigment particles.

Testing Handmade Dispersions

My own experiments with pigment grinding focused on two difficult pigments we work with at Tri-Art manufacturing: raw umber and quinacridone magenta. Quinacridone red is modern, synthetic, dye-like pigment. Its small organic particles would rather stick to each other than disperse into paint. I could tell just by looking at these pigment powders under magnification that I had some significant clumps that I would have to work out in this paint.

two containers of pigment, one red, one brown.

Figure 8: Quinacridone red pigment, raw umber pigment.

Watercolours are particularly suited to hand mulling. The (re)solubility of gum Arabic means that water can simply be dropped back onto the plate as the paint is being dispersed by the action of the muller. Here you can see that even with an initial mulling my raw umber paint has a gritty texture to it. Both paints at this stage were also lacking saturation, seen in my paint outs in the next figure. With successive mulling, the paints became smooth and pleasantly thick watercolour. I painted out more swatches as I went, watching the colour saturation increase as the pigment particles were dispersed.

Samples of watercolour during mulling with a glass muller.

Figure 9: Left, Mulled watercolour medium and raw umber. Right, After successive mulling, smooth, quinacridone red watercolour.

Paint outs of watercolours, both brown and red. Three samples each show the watercolour becoming more saturated and even in colour.

Figure 11: Good, better, best. Watercolour dispersions.

Despite several minutes of mulling the paint mixture, some particles are still apparent in the paint out of raw umber, but overall, I was very happy with results. Hand mulling pigment and watercolour medium resulted in a richly pigmented paint that showcased the vivid and chromatic quinacridone red and brought out the coolness of the raw umber. With a successful start, I turned my attention to the real challenged, could I mix a workable paint with pigment and acrylic medium?

Final images of watercolours made by hand dispersion with glass muller. Two pans of red and brown and two paint outs.

Figure 12: Final paint outs of handmade watercolour. Raw umber still shows undispersed particles.

Mixing raw umber pigment directly with acrylic medium with just a palette knife produced a gritty paint that very quickly highlighted for me the difficulties of grinding pigment directly into acrylics. The medium dried so quickly that little time was available for me to work the pigment into the paint. Adding more medium allowed me a longer working time, and with this I achieved a slightly less gritty texture, but not one that I would deem successful by any means. I knew that if the inorganic particles of iron oxide I could see were not being dispersed, the much smaller carbon black for sure were still largely clumped together as well.

a palette knife, acrylic medium, and dry umber pigment in the first image is mixed into a slightly gritty paint in the second.

Figure 13: Hand dispersing dry raw umber into acrylic medium. Right, First attempt at directly grinding dry pigment into acrylic medium.

An image of brown acrylic paint after further hand mixing and medium added, the paint has smoothed out.

Figure 14: Results of further mixing and addition acrylic medium added (dry pigment directly into acrylic medium).

Despite being a much finer particle, quinacridone red didn’t fare much better. I needed a liquid dispersion that I could grind then incorporate into the fast-drying acrylic. With my fresh, successful watercolour at hand I attempted a slightly unconventional pairing. A mixture of watercolour and acrylic produced a richly pigment paint film, one that seemed promising at first. After allowing my paint to dry, I notice both films had a considerable concentration of aggregated particles – perhaps pigment, perhaps slightly dried medium from the mixing process. The glossy richness faded, leaving a dull matte surface that no longer showcased the vibrancy of my quinacridone red. While certainly not a combination I would ever advise for a working artist, the experiment solidified for me the difficulties of working pigments and water-based acrylic mediums.

A sample of red paint, photographed in normal and raking light to showcase both the saturation of colour, but also the particles still apparent.

Figure 15: Quinacridone red handmade acrylic paint. The right image in raking light shows the aggregated particles that formed in the paint.

Back to the Grind: The Everyday Takeaways of Pigment Grinding

It may require the careful finesse of an expert paint maker, but the benefits of properly ground pigments are clear. An excellent dispersion allows for the formulation of paint that is smooth (it never should be gritty) while balanced with a range of high to low gloss specific to each pigment. Furthermore, it creates paint with a consistent mass- and undertone that allows for tinting and colour mixing without a tendency towards a lack of chroma or muddy hues. If you find that a drop of titanium white turns you colours ‘beyond the pale,’ that your colours lack saturation, your glazes are hazy, or your paints have little covering power, you may want to consider evaluating the quality of your paints – especially when it comes to acrylics.

When selecting acrylic paint, be sure to choose only artists quality acrylics, made by a reputable brand that stand by their products. If budget is a consideration for your work, be sure to do your homework before purchasing what is simply within your reach. Pigments, by a lion’s share, are the most expensive part of any paint formulation. Manufacturers that take the time to fully grind and disperse pigments can utilize less pigment to produce their budget lines, while retaining quality of colour, without the use of cheap fillers. Not only can expert pigment grinding amount to savings on the upfront cost of a budget quality line, but also the cost-per-use, as these are generally higher quality products with more saturation and staining power. By mastering pigment grinding for our own finest quality artists lines at Tri-Art, we are also able to offer a high quality student and kids line, at a very competitive price as well.

blue high viscosity paint on a paint brush

[1] Wetting and Dispersing Additives (BYK Additives and Instruments).

[2] George O’Hanlon, ‘Traditional Oil Painting: The Revival of Historical Artists’ Materials – Natural Pigments’, Natural Pigments, 2013 <https://www.naturalpigments.com/artist-materials/traditional-oil-painting-revival/> [accessed 19 January 2021].

[3] Antti Mäntynen and others, ‘Optimization of Grinding Parameters in the Production of Colorant Paste’, Powder Technology, 217 (2012), 216–22 <https://doi.org/10.1016/j.powtec.2011.10.029>.

[4] Wetting and Dispersing Additives.

[5] Christiana Agbo and others, ‘A Review on the Mechanism of Pigment Dispersion’, Journal of Dispersion Science and Technology, 39.6 (2018), 874–89 <https://doi.org/10.1080/01932691.2017.1406367>.

[6] Mäntynen and others.

[7] Mäntynen and others.

[8] Wetting and Dispersing Additives.

[9] Wetting and Dispersing Additives.

Bibliography

Agbo, Christiana, Wizi Jakpa, Bismark Sarkodie, Andrews Boakye, and Shaohai Fu, ‘A Review on the Mechanism of Pigment Dispersion’, Journal of Dispersion Science and Technology, 39.6 (2018), 874–89 <https://doi.org/10.1080/01932691.2017.1406367>

Mäntynen, Antti, Alexey Zakharov, Sirkka-Liisa Jämsä-Jounela, and Mats Graeffe, ‘Optimization of Grinding Parameters in the Production of Colorant Paste’, Powder Technology, 217 (2012), 216–22 <https://doi.org/10.1016/j.powtec.2011.10.029>

O’Hanlon, George, ‘Traditional Oil Painting: The Revival of Historical Artists’ Materials – Natural Pigments’, Natural Pigments, 2013 <https://www.naturalpigments.com/artist-materials/traditional-oil-painting-revival/> [accessed 19 January 2021]

Wetting and Dispersing Additives (BYK Additives and Instruments)

Categories
Education Notes from the Lab

Modern and Post-Modern Pigments: Cadmiums vs. Bismuth Yellow and Pyrrole Red

Painters today are offered a considerable breadth of choices when selecting paints. It’s easy to become overwhelmed. We might default to using the pigments our instructors taught us to use or those we have simply used for many years. The wide accessibility and relative affordability of so many pigments is nothing short of modern miracle, sustained on by a global system of trade and manufacture. Rather than fear the long shelves of unfamiliar pigments I hope I can convince you that these options are often solutions to our artistic aims.

Those new to painting may search ‘essential paint colours’ and find most lists include some variation of cadmium yellow and red. These are the iconic, bright, opaque colours that have been loaded onto painters’ palettes for nearly two centuries now. Today’s article makes the case for cadmium’s post-modern ancestors, bismuth yellow and pyrrole red. Where did these pigments come from and why should you choose them instead of cadmium colours?

drawdown swatch of yellow and red

Figure 1: Cadmium Yellow (Medium, PY 35), Bismuth Yellow (Medium, PY 184), Cadmium Red (Medium PR 108), Pyrrole Red (opaque, PR 254)

Heavy Hitters: A history of Cadmium Pigments

Although they may seem firmly historical, heavy metal cadmium pigments (red, yellow and orange) actually have somewhat recent history, but certainly one that has made a prolific splash. The critical elements of these iconic colours, cadmium and selenium, were not discovered until 1817.[1] When the German metallurgist Friedrich Stromeyer discovered cadmium in 1817, he immediately recommended his brightly coloured cadmium sulphide compound for use as an artist’s pigment. It would take the work of another chemist, the Swedish Jöns Jacob Berzelius, with his discovery of selenium, to make orange and red shades of cadmium with the addition cadmium selenide to Stromeyer’s cadmium yellow.[2] Cadmium pigments now also contain a portion of zinc sulphide which in combination with yellow cadmium sulphide which yields cool, light hues of colour like cadmium yellow primrose.[3]

a row of cadmium paint swatches from yellow to red

Figure 2: Tri-Art High Viscosity Clinically Pure Cadmium Colours: Yellow Light (PY 35, with the most zinc sulphide), Yellow Medium (PY 35), Orange (PO 20, with increasing cadmium selenide), Red Medium (PR 108, with the most cadmium selenide).

Cadmium pigments appears as early as 1829 in oil paintings in France and Germany but were slow to take off. Cadmium pigments were not commercially available until 1840[4] with a very limited supply of raw materials – to this day there are no readily accessible sources of cadmium and selenium, and so these components must be processed out of other mining waste. [5] Scarcity persisted throughout the 19th century with one first-hand account from 1888 noting cadmium yellow as “a perfect colour if not so expensive.” [6]

Despite their cost, these pigments gained popularity for their enduring colour, especially in the heavily polluted air of 19th century coal-burning cities.[7] They found use in oils and watercolours, where small amounts of the costly pigments could be appreciated. The famous colours of the Impressionists certainly owe a great dept to the bright, opaque shades of cadmium colours that added chroma to their a la prima palettes. Monet’s works have been extensively documented to contain cadmium yellow, as seen in the warm yellow hues of Bordighera (1884).[8]

a painting, bordighera by claude monet

Figure 3: Claude Monet, Bordighera, 1884, The Art Institute of Chicago, Illinois. Public Domain, circa wiki Images.

The 1920s brought the industrialization of cadmium pigments. It was discovered that cadmium colours could be extended with inexpensive lithopone filler while still remaining colourful and opaque. With this, cadmiums became one of these most important commercial pigments, still being produced in mass quantities to this day. [9] Quality artists’ paints continued to use unadulterated cadmium colours to capitalize on the vibrancy of these pigments, but at a premium cost. The ubiquitous nature of these colours on nearly any painter’s palette is a testament to their beloved working qualities, and perhaps lack of alternatives.

Bismuth as Usual? The Long History of A Post-modern Yellow

You may be familiar with bismuth metal in its crystallized state, an impossibly geometric and prismatic structure, commonly sold in shops. While these are modern synthetic crystals, bismuth as a white metal was known to the ancients and found use in artworks as early as the 15th century. The metallic quality of bismuth meant it could be utilized as an economical substitute for silver, most commonly found as a powdered pigment for illuminating of manuscripts or oil paintings.[10] The pigment was likely more grey than metallic, with a slight lustre similar to metal-point drawings or modern graphite. Painters like Francesco Granacci (1469–1543) may have tried to utilize this lustrous quality when painting metallic surfaces like the armour in Portrait of a Man in Armour (ca. 1510). Modern analysis found the grey to be bismuth based.[11]

synthetic bismuth crystal

a painting, a portrait of a man in armor

Figure 4: Modern Synthetic Bismuth Crystal by Dschwen. CC 2.5. Via Wiki Images.
Bismuth Metal Ingots. By Unconventional2 – Own work, CC BY-SA 4.0, via Wiki Images.
Francesco Granacci (1469–1543), Portrait of a Man in Armour (ca. 1510). National Gallery of Art, Washington DC. Public Domain, via Wiki Images.

Metallic bismuth as a pigment does not appear to have gained wide traction in history, and only in the 20th century was the metal explored for its colorful possibilities. Beginning in the 1960s the spectral effect of bismuth crystals was capitalized on in the form of a nail polish additive – bismuth oxychloride was utilized as a replacement for naturally pearlescent materials.[12] Contemporary bismuth yellow pigments were only introduced to market in 1985 after many decades of experimentation.[13]

3 bismuth yellow colour swatches

Figure 5: Post-modern yellows. Tri-Art Liquids Bismuth Yellow (PY 184) in Light, Medium and Deep Hues.

Bismuth Yellow (labeled PY184 on artists paints) is a mixture of two metal oxides, bismuth vanadate and bismuth molybdate. Bismuth vanadate was first synthetized in 1924 for pharmaceutical purposes. In 1976 DuPont began developing this compound as a pigment. They described their bismuth vanadate as ‘brilliant primrose yellow.’ Mixed metal oxide version of vanadate and molybdenum were later synthesized, giving rise to warmer orange-red shades.[14] Tri-Art offers shades from light to deep, similar to shades of cadmium yellow. Approximately 900 tonnes of bismuth yellow are now produced annually, [15] largely for industrial, outdoor applications due to its outstanding lightfastness.

Pyrrole Reds: A Fiery Future

Pyrrole red is a relatively new pigment – it was first synthesized in 1974 by accident. [16] Despite its infancy, many of us will have encountered this pigment in everyday life, the pigment has become widely used in automobile paints, plastics and cosmetic formulations (labeled CI 56110).[17], [18]

two pyrrole red swatches

Figure 6: Tri-Art Liquid Acrylic Pyrrole Red Light (PR 255) and Medium (PR 254).

When pyrrole red was first synthesised, the accidental biproducts had enticing properties as pigments: highly insoluble, highly stable, and intensely red in colour. The product had an incredibly high chroma red colour due to their synthetic, high purity.[19] The pigment was first introduced as Irgazin DPP in the 1980s by Ciba-Geigy.[20] It now comes in a range of hues from orange to bluish-red.[21] Tri-Art offers pyrrole red light and medium in similar shades to cadmium reds.

Modern vs. Post-Modern pigments: Which are better?

Cadmium pigments are considered staple of the serious painter’s palette – intensely opaque, they offer great colour coverage and tinting strength, especially when mixing with titanium white. However, cadmium colours have always been cost prohibitive and will continue to be so due to the scarcity of available cadmium and selenium on earth. Cadmium pigments also have record of causing problems for painters and conservators. In their 19th century infancy cadmium pigments were particularly unreliable; contaminants left over from the processing of cadmium pigments from raw ore could cause disastrous deteriorations to painted surfaces in just a few decades. Many 19th century paintings with once bright, richly saturated yellows now appear bleached white, cloudy, or chalky in appearance. Several cases show the opposite effect too, with mixtures of cadmium yellow and other pigments like chrome yellow and the famous Emerald Green (composed of deathly toxic arsenic) inciting reactions that markedly darken the paint film. It really took the improvements of 20th century chemistry to bring us chemically pure cadmium pigments that were consistently stable in the 1940s.[22]

Despite finding stability in chemically pure formulations, cadmium pigments remain problematic because of their heavy metal nature. Toxicological and environmental impacts are something that every painter should consider when deciding if they want to use these pigments. Because of these issues, pyrrole red and bismuth yellow are often suggested as replacements for these pigments. Pyrrole red is nearly just as opaque as cadmium red, intensely red, outstandingly light-fast, and non-toxic. [23] Because of its safety profile, the pigment is safe enough to be used in tattooing, cosmetics and all paint formulations. Unlike cadmium colours, post-modern pigments like pyrrole red and bismuth yellow can found in liquid and spray paints – cadmium colours, in contrast, should not be sprayed to avoid any inhalation.

cadmium red swatches next to pyrrole red swatches

Figure 7: Tri-Art High Viscosity Acrylics. Cadmium Red (Medium Shade, PR 108) and Pyrrole Red (medium shade, PR 254). Straight from the tube, as a tint with titanium white (PW 6), and as a wash with water.

Bismuth yellow is similarly non-toxic, with a very high opacity, and an intense yellow colour. Although a metal oxide, it appears that bismuth yellow is an environmentally conscious choice with impact studies noting virtually no risk to human or aquatic life.[24] Bismuth yellow also has excellent lightfastness, being used extensively for outdoor applications.[25] Like cadmium pigments, bismuth yellow is an expensive pigment, owing to the cost of raw materials. Like cadmium, bismuth is somewhat scarce and therefore can only be sourced as a biproduct of mining other metals.[26]

cadmium yellow swatches next to bismuth yellow swatches

Figure 5: Post-modern yellows. Tri-Art Liquids Bismuth Yellow (PY 184) in Light, Medium and Deep Hues.

Pyrrole red has the additional advantage of coming in a range of not only shades, but also transparency. Unlike cadmium colours, the particle size of pyrrole red can be manipulated to make transparent and opaque formulations. Painters may be more familiar with this colour than they think – transparent pyrrole red has been used to create the red filter of RGB type LCD screens.[27] Although these screens project colour rather than reflecting it as in a painting, modern artists may find they can reproduce the high chroma colours we see every day through our screens more faithfully using contemporary pyrrole pigments versus muddier cadmium colours. Transparent pyrrole reds are additionally more lightfast than many other transparent reds that are used to formulate colours like alizarin crimson.

red drawdown colour swatch

Figure 9: Tri-Art Liquid Acrylics, Transparent and Opaque Pyrrole Red, Medium Shade (both PR 254).

a macro example rgb pixels displaying different colours

Figure 10: LCD screen showing individual pixels of red, green and blue. The red filter can be made using transparent pyrrole red. By Luís Flávio Loureiro dos Santos, via Wiki Images. CC 3.0.

Testing out Bismuth Yellow, Pyrrole Red and Cadmium Colours

Advantages of Pyrrole Reds:

  • Pyrrole red is non-toxic, and more ecologically friendly than cadmium colours.
  • Despite its price point, the very high tinting strength of Pyrrole red allows this colour to go very far – Tri-Art’s high quality, highly pigmented, professional formulations further ensure you get the most pigment and colour possible.
  • Pyrrole colours are the closest alternative to cadmium colours rather than cadmium hues currently on the market.[28] Hue colours must balance an accessible cost with replicating the hue of another pure pigment. If you are looking to replace heavy metal pigments like cadmiums from your palette at a professional level – pyrrole is the best choice.
  • Pyrrole red creates cleaner secondary colours, less warm and muddy than cadmiums. Its high chroma also competes better with other high chroma colours like phthalo blue and high tinting colours like titanium white.
  • Pyrrole red has options for opaque and transparent variations – if you can become familiar with its colour properties you can readily extend your working palette to transparent reds rather than reaching for other hues of red.

cobalt and magenta paint swatches

Figure 11: Cobalt Blue and Manganese Blue Hue (phthalo blue) mixed with cadmium red (left row), pyrrole red (right row), and the resulting mixtures with titanium white (bottom row). Pyrrole red created a richer purple with both blues, while cadmium produced warmer, muted hues. When up against titanium white, pyrrole red colours remained more saturated in chroma, with cadmium colours becoming even more muted.

bismuth, cadmium, and pyrrole swatches on two cards

Figure 12: Bismuth Yellow (left) and Cadmium Yellow (right) mixed with cadmium red (left row), pyrrole red (right row), and the resulting mixtures with titanium white (bottom row). Results were similar to mixing purple secondaries, with pyrrole red producing richer, more saturated colours, even when mixed with titanium white.

Advantages of Bismuth Yellows:

  • Bismuth yellow is ecologically more friendly than cadmium colours and non-toxic.
  • Like pyrrole red, bismuth yellow creates cleaner secondary colours. I found bismuth yellow most convincing when mixing green hues – it readily produced more neutral greens than cadmium yellow that always appeared yellow and warm shaded.
  • Last year Tri-Art introduced a newly modified bismuth pigment which improved grinding and dispersing for their bismuth yellow acrylic paint. The new pigment creates a paint with higher gloss, greater chroma and saturation, and requires a less energy intensive process to create.

cobalt and manganese swatches

Figure 13: Cobalt Blue (left) and Manganese Blue Hue (phthalo blue)(right) mixed with cadmium yellow medium (left row), bismuth yellow medium (right row), and the resulting mixtures with titanium white (bottom row). Bismuth yellow created a more neutral green when mixed with both blues, while cadmium produced warmer, yellow hues.

By now all but the traditionalist painter might be convinced of these contemporary pigments. But for those who would like to replicate the works of 19th and 20th century geniuses I would offer one last thought. While making my colour charts, I also found that bismuth yellow was able to make a beautiful shade of bright and cool green with phthalo blue and titanium white – the colour reminded me of the infamous Emerald Green. Thankfully no longer available due to its incredibly toxic arsenic content, the bright and densely opaque green colour dominated the art world of the 19th century. Degas possibly used the pigment when painting his ­Dancer with Bouquets (1895-1900), adding bright flourishes of cool green. Whatever your artistic aims, I would suggest that pigments like pyrrole red and bismuth yellow offer painters, both modern and classical alike, the ability to mix a wider range of colours while retaining vibrancy and saturation when creating tints with titanium white. Consider trying them for your next project.

painting of ballerina on stage with bouquets at her feet, showing use of emerald green

Below, Bismuth Yellow, phthalo blue and titanium white mixtures

Figure 14: Edgar Degas, Dancer with Bouquets, 1895-1900, oil on canvas. Chrysler Museum of Art. Public domain. Below, Bismuth Yellow, phthalo blue and titanium white mixtures. The bright touches of green in this work are possibly the infamous arsenic based emerald green, a colour that can no longer be obtained. Replicating the colour with modern pigments like bismuth yellow worked better than cadmium colours.

When to reach for Cadmium Reds and Yellows:

Despite their drawbacks, cadmium colours will continue to hold an important place in the painters’ palette. For the contemporary painters looking to evaluate their painting practice you may want to consider using cadmium colours for the follow purposes:

  • Trying to replicate a historical palette (note that many historical colours are not available due to their toxicity or fugitive nature)
  • Opacity is critical (pyrrole red and bismuth yellow are slightly less opaque than cadmium colours)
  • You are looking to achieve a palette that is warm or low in chroma while keeping colour mixtures minimal
  • For those on a budget, cadmium hue colours can be explored too. However, these will be most useful for those who paint straight from the tube, without extensive colour mixing.

Bibliography

Abel, A, ‘Pigments of the Industrial Revolution’, in Colour Design – Theories and Applications, ed. by Best Janet, 2nd Editio (Cambridge: Elsevier, 2017), pp. 572–74

Berrie, Barbara H, ‘Rethinking the History of Artists’ Pigments Through Chemical Analysis’, Annual Review of Analytical Chemistry, 5.1 (2012), 441–59 <https://doi.org/10.1146/annurev-anchem-062011-143039>

Buxbaum, Gunter, ed., ‘Bismuth Pigments’, in Industrial Inorganic Pigments, Wiley Online Books (Weinheim: Wiley, 1998), pp. 113–16 <https://doi.org/https://doi.org/10.1002/9783527612116.ch3>

Čechák, Tomáš, Tomáš Trojek, Radka Šefců, Štěpánka Chlumská, Anna Třeštíková, Marek Kotrlý, and others, ‘The Use of Powdered Bismuth in Late Gothic Painting and Sculpture Polychromy’, Journal of Cultural Heritage, 16.5 (2015), 747–52 <https://doi.org/10.1016/j.culher.2014.12.004>

Dunning, Paul, ‘Cadmium Pigments’, High Performance Pigments, Wiley Online Books, 2009, pp. 13–26 <https://doi.org/https://doi.org/10.1002/9783527626915.ch3>

Endriss, Hartmut, ‘Bismuth Vanadates’, in High Performance Pigments, Wiley Online Books, 2009, pp. 7–12 <https://doi.org/https://doi.org/10.1002/9783527626915.ch2>

Fiedler, Inge, and Michael Bayard, ‘Cadmium Yellows, Oranges and Reds’, in Artists’ Pigments: A Handbook of Their History and Characteristics, ed. by Robert L Feller (Washington DC: National Gallery of Art, Washington DC, United States, 1986), p. pp.65-108

Greening, Timothy, ‘Metamerism in Colour Mixtures Containing Cadmium Red and Pigment Red 254’ (Queens University, 2013)

Krüger, Joachim, Peter Winkler, Eberhard Lüderitz, Manfred Lück, and Hans Uwe Wolf, ‘Bismuth, Bismuth Alloys, and Bismuth Compounds’, Ullmann’s Encyclopedia of Industrial Chemistry, Major Reference Works, 2003 <https://doi.org/https://doi.org/10.1002/14356007.a04_171>

Lewis, Peter A., ‘Colorants: Organic and Inorganic Pigments’, in Color for Science, Art and Technology, ed. by Kurt. Nassau, Azimuth ; v. 1. (Amsterdam ; Elsevier, 1998), pp. 283–312

Lomax, Suzanne Quillen, and Tom Learner, ‘A Review of the Classes, Structures, and Methods of Analysis of Synthetic Organic Pigments’, Journal of the American Institute for Conservation, 45.2 (2006), 107–25 <http://www.tandfonline.com/doi/abs/10.1179/019713606806112540>

Wallquist, Olof, and Roman Lenz, ‘Diketopyrrolopyrrole (DPP) Pigments’, High Performance Pigments, Wiley Online Books, 2009, pp. 165–94 <https://doi.org/https://doi.org/10.1002/9783527626915.ch11>