Calcite Oil Painting Mediums

Calcite is a naturally occurring calcium carbonate (CaCO₃) mineral chiefly found in rocks, such as chalk, limestone and marble. These rocks are the main sources for the pigment. Its whiteness, softness and fine-grained nature makes it an ideal white pigment, being both abundant and easily processed. Chalk is relatively transparent in most paint media and is therefore often used as an extender of other pigments and employed with animal glue as a ground for painting.

History of Calcite in Art

Calcium carbonate in various forms has a long history in art. It has been recognized, for example, in Greek and Roman art. Its use in art since that time has been both persistent and widespread. In northern Europe, from medieval times chalk was employed with animal glue for making the ground or preparation layer of paintings. Sometimes later lead white was mixed with the chalk to make it denser and whiter. Chalk was used with animal glue or with other aqueous binders as a white pigment.

Calcite has been used in oil painting, being added to colors, especially flake white, by such artists as Velázquez and Rembrandt. The transparency of chalk was desirable in some Dutch tonal landscapes of the seventeenth century.

Uses of Calcite in Paint

Calcium carbonate is commonly used in combination with other pigments in oil paint because of its low refractive index and hence poor hiding power. Linseed oil and chalk have long been used together, however, in the preparation of putty. The transparency of chalk in oil makes it ideal for adding bulk to oil colors or to affect the consistency (rheology) of paint.

Calcium carbonate is often used as an extender pigment to reduce the cost of paint. Its alternative names, such as chalk and whiting, are used in many formulations. An important use of calcium carbonate is in mixtures with titanium dioxide pigment to act as a spacer, keeping the titanium white particles spread apart, and ensuring better efficiency in scattering light and increasing opacity. Calcium carbonate is often used to vary the gloss of powder coating materials, depending on the particle size used.

Variations of Calcite and their Effects on Paint

Calcium carbonate derived from different mineral sources behave differently in paint. The material can be ground from limestone, a sedimentary rock formed in sea beads or alluvial deposits; or marble, which is limestone that has undergone heat and pressure below the earth's crust; or chalk, a light, low structure material normally associated with the sedimentary deposition of the shells of such minute marine organisms as foraminifera, coccoliths and rhabdoliths. The particle structure and chemical behavior of these variations of calcite all differ slightly.

The particle size and shape of calcite in regards to the behavior and performance of paint are important for several reasons. One is the viscosity of the paint, which is related to the volume occupied by the dispersed solids within the paint vehicle. In the case of particles that are not spherical, the “spherical equivalent” volume may be the maximum volume inscribed by the rotation of a particle. Because of this, a non-spherical particle may behave as if it occupies much more volume than it does.

Another consideration of the influence of particle size and shape on the behavior of paint is the surface area of the particle. The amount of paint binder required by a pigment to form a paste paint is called its oil absorption number. The greater the surface area of the particle, the more binder it demands to make into a paste or flowing paint. Synthetic (precipitated) calcium carbonate that consists of 0.05 micron needle-shaped particles has more surface area than the particles of ground limestone of the same size, which have simple structures resembling rhomboidal crystals. Because of the complex surface of the precipitated calcium carbonate, it will tend to scatter more light and consequently appear more opaque than the ground limestone particles. However, it is likely that this precipitated calcium carbonate will consume considerably more binder than the ground limestone.

Selecting the right type of calcite, taking into consideration particle size and shape, brightness, chemical constituents, and surface treatment are important factors when it comes to making paint or oil painting mediums.

Rublev Colours® Impasto Medium

Impasto Medium is an oil painting medium of finely ground calcite, silica and bentonite in bodied linseed oil. Use it to extend paint without altering the consistency, making oil colors 'short' and buttery for impastos. Stiffer than Velázquez Medium, this putty-like medium makes colors slightly transparent while allowing you to build impasted, thick applications of paint. When added to oil paint, Impasto Medium does not alter its color temperature. It can be thinned with solvent or oil. Depending upon the proportion added to paint, Impasto Medium does not affect the drying time significantly.

Impasto Medium does not contain stearates, solvents, driers or natural or synthetic resins, so it is safe to add to oil paint without the worry of cracking or delaminating.

Impasto Medium is a calcite-rich oil painting medium.

Impasto Medium slightly increases the transparency of colors while maintaining the buttery consistency. It allows you to build impasto or thick paint with ease. When added to oil paint, brushstrokes hold their form without slumping or leveling. In the picture below, when added to green earth artist oil (bottom center), the mixture of Impasto Medium (bottom left) extends the color and its consistency, while the mixture with Velázquez Medium (bottom right) gives the paint a long, ropy consistency.

Rublev Colours® Velázquez Medium

Velázquez Medium is an oil painting medium of finely ground calcite in bodied linseed oil. Use it to extend paint and alter its consistency, making oil colors 'long'* for finer detail and impastos. Softer than Impasto Medium, this flowing paste makes colors slightly transparent while allowing you to build impasted, thick applications of paint. When added to oil paint it does not alter the color temperature. It can be thinned with solvent or oil. Depending upon the proportion added to paint, it does not affect the drying time significantly.

Velázquez Medium does not contain stearates, solvents, driers or natural or synthetic resins, so it is safe to add to oil paint without the worry of cracking or delaminating.

*Long refers to the consistency of paint that displays tall peaks when a palette knife is pressed to its surface and lifted. Long paint is also described as being a "ropy" and refers to a stringy quality, like that of honey. Paint that behaves in this way is said to have long rheology. Short rheology refers to paint that exhibits a more buttery consistency, typical of most commercial oil paint. Rheology is the study of how substances flow, such as liquids and soft solids that flow rather than deform elastically.

Velázquez Medium is a pure calcite oil painting medium.

The soft white paste has little affect on color, making them less opaque and slightly less saturated. Use it to build thick applications of paint.

Velázquez Medium slightly increases the transparency of colors while introducing flowout and leveling. These properties allow you to create fine detail in impasto or thick paint with ease. Although the paint becomes long when Velázquez Medium is added, it holds brushstrokes well. In the picture below, when added to green earth artist oil (bottom center), the mixture of Impasto Medium (bottom left) extends the color and its consistency, while the mixture with Velázquez Medium (bottom right) gives the paint a long, ropy consistency.

A comparison of Impasto Medium (top left) and Velázquez Medium (top right) shows the different handling characteristics when added to paint (bottom left and right).

How to Use

Mix Rublev Colours Calcite Mediums directly into your paint right on the palette with a palette knife.

Rublev Colours Calcite Mediums are a great way to add calcite to your own paint. Add the medium with linseed oil directly to pigment powder until a stiff paste is obtained (almost the consistency of putty). Grind the paste with a muller on a flat surface before storing in collapsible tube. Some of its properties will be temporarily lost when grinding the medium with a muller, but should be restored when allowed to stand.

Rublev Colours Impasto Medium

Rublev Colours Velázquez Medium

Most-Asked Questions and Answers

As calcium carbonate, how is calcite different from using marble dust?

Calcite is the calcium carbonate mineral found in such rocks as chalk and marble, which both are varieties of limestone. The difference between calcite from marble and that from chalk deposits is the crystalline structure and the minerals often associated with each type of rock. Marble is an angular crystalline variety of limestone that differs in its shape to chalk. For this reason, marble is more often ground coarse and hence used to impart texture to products. However, comparable grades of calcite from marble or chalk can be found today. The quality of the marble dust or chalk should be evaluated based on the fineness of its particle size and the whiteness of the material.

I would like to use calcite ground or mixed with a knife in oil or medium as an additive to make paint transparent. Can I make it myself using the raw materials?

Yes, you can make your own calcite mediums or add calcite directly to paint and we encourage artists do to so by providing all the raw materials needed. The two calcite mediums offered by Natural Pigments, however, provide a convenient way to use calcite in your painting without the trouble of grinding the powders and mixing the oils. In addition, Natural Pigments has carefully formulated the mediums to provide widely different painting characteristics, which would be difficult to achieve without much trial and error.

Rublev Colours® Artists' Oils

Rublev Colours Artists' Oils let you experience what the old masters well understood—the unique characteristics of pigments. The pigments used by old masters in their paintings were ground from natural minerals and earths, fermented in dyer vats and concocted in alchemist laboratories. Rublev Colours Artists' Oils give you the same pigments used by the old masters prepared with linseed oil as ready-made paints.

Most oil colors today are made to feel the same under the brush. Their consistency is short and buttery, irrespective of the color. Whereas a short and buttery consistency is good, there are many times when you want paint that has a different feel. Sometimes you need paint that flows. Other times you want a long and perhaps even ropy paint. Or one that flows when brushed then thickens upon standing, which is called thixotropic paint. That is why so many painters today resort to using mediums with their tube oil colors—to alter the consistency of their paint.

The Rublev Colours Difference

Why are Rublev Colours different from other commercial oil colors? One reason is that we use natural pigments or historical reproductions of pigments used by the old masters. Another reason is that we make Rublev Colours Artists’ Oils as they did before modern tube colors—without additives. Rublev Colours Artists’ Oils are formulated to maintain the unique characteristics of each pigment in oil. The character found in each tube of our oil colors is unique due to the pigment inside, giving the artist nearly limitless choices of texture, opacity, consistency, tone and hue. With Rublev Colours you experience the transparency of yellow ochre, the pale coolness of green earths, and the crystalline glitter of deep blue azurite.