Calcite--Types, History and Use in Oil Painting
Calcite is a naturally occurring calcium carbonate (CaO3) 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.
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.
posted by George O'Hanlon at
9:38 PM
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