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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)