Iron dyes - natural and artificial

Iron dyes

The dyes found in this group are divided into natural and artificial.
The natural ones are called earthen and ochre and consist mainly of clay colored with aqueous or anhydrous iron oxide.

Properties:
1. absolute permanence to light.
2. non-poisonous.
3. inexpensive.
4. Used in all techniques.

Iron dyes - natural

Yellow ochre

Chemically - clay colored with aqueous iron oxide.Depending on the intensity of the color, they are divided into light, dark, golden and orangish. Lightfast, they behave well in mixtures with other dyes. Used in all techniques. Due to their high clay content, they are among the coarse-grained, but cover well.
They need 40 to 60% oil. They belong to medium-drying paints.

Orangish and golden ochre

Contains a higher amount of aqueous iron oxide than light and therefore covers slightly less well than light. Belongs to semi-laser dyes. It needs about 70% oil and dries more slowly.
Permanent. Often colored with yellow chrome or aniline pigments.

Dark ochre

With a higher iron oxide content.
They also have a certain amount of manganese, so they dry more quickly.
They are durable, but in oil technique they often darken due to the higher amount of oil used in rubbing and the content of bituminous bodies.

Roasted ochers
They are obtained by roasting yellow ochers to give them a brown or red color.
These include Venetian, Pompeian and Puccola reds.
The color depends on the heat and purity of the raw material.
From dark ochers, brown ochers are obtained in the same way.
By roasting, ochre loses its chemically bound water and changes its color, but retains its properties.
Recognition:
Yellow and red tangs do not dissolve in alkali, spirit and water. In dilute hydrochloric acid they dissolve partially, coloring the solution yellow.
The clay parts give a precipitate. When heated, they darken.
Burnt red ochre dissolves in dilute HCl. The color of the solution is yellow. When heated, they do not change color.

Siena

This is a dye belonging to the order of earth dyes - ochres, although it differs significantly from them in chemical composition. It is an iron silicate salt.
Instead of clay, it contains silicon colored with aqueous iron oxide. Due to its chemical composition, sienna belongs to laser dyes. When rubbed, it needs an oil of 159 to 241%, which is why it darkens and dries slowly in the oil technique.
With other properties it is similar to ochre. The best sienna is obtained in Italy, near Siena - hence the name.

Burnt sienna

It has a reddish-brown color and is characterized by better covering power.
Roasted Siena is used in all techniques.
Recognition:
Natural Siena dissolves when boiling in concentrated hydrochloric acid, forming a jelly-like mass with an intense yellow-brown color.

Red ochers

Red ochers differ in their composition from yellow ones only in that they are colored with anhydrous iron oxide.
They occur in volcanic gesso and are immediately roasted. Many of them, such as rubric and bolus, were known in ancient times. They are mined mostly in Italy.

Sanguine

Clay + anhydrous iron oxide. It is used in the manufacture of special pencils.

Bolus

Depending on where it is mined, it has a different chemical composition. Sometimes it is silicon-iron salt (like sienna), sometimes clay colored with iron oxide (anhydrous). In the 18th century, bolus was widely used primarily in gesso.

Indian red

Contains a large amount of iron oxide - hence the intense red color.

Puccola

Has a complex chemical composition. In addition to clay and iron oxide, it contains calcium, magnesium and sodium.
All red ochres are durable and used in all techniques. They dry well in oil, and need 45 to 50% of it for rubbing.

Green earth

It consists of clay, silicates, oxides of iron, manganese, calcium and sodium. These elements are chemically bound to each other in the dye.These components, depending on where the dye comes from, occur in different proportions, which is reflected in the hue and properties.
The dye was known in ancient times and is still used readily today, and is mined in Italy, Tyrol and Cyprus. The best grades come from under Verona and this is where the name "Veronese earth" (olive shade) comes from.
Green earth is durable, used in all techniques. It is especially valuable in fresco technique, where it is used to better fix certain pigments.
In oil it dries at a medium speed and needs 100% of it.
Green earth stored for a long time in its natural state (powder) with access to air - it browns (iron oxide absorbs oxygen and turns into a higher degree of saturation - that is, iron oxide).
By burning, it turns brown and is known as "roasted green earth."
More recently it has been used to make green seal wax.
Recognition:
The imitation of green earth by mixing ugre with berlin azure is detected by the action of alkali on azure. The coloring with organic dyes is learned by a test with spirit. The presence of copper ones is detected with acetic spirit, with which they give a blue solution.

Umbra

Similar in chemical composition to ochre, but contains larger amounts of aqueous manganese oxide, which gives it a brown color. Durable, it dries quickly in oil (manganese), although it needs it in larger amounts (100%).
Used in all techniques, both natural and roasted (reddish brown).
Known in ancient times, mined in Italy in Umbria.
Recognition:
When umber is heated with hydrochloric acid, it produces chlorine with a characteristic odor.
In alkalis it does not dissolve.

Iron dyes - artificial

Marshes

Yellow mars are ugrams obtained artificially. They can consist of pure aqueous iron oxide, but can also contain clay, gypsum, zinc oxide and chalk in their composition.
One of the simpler ways of obtaining yellow mars is by mixing a solution of iron kynopoc with milk of lime. The greenish precipitate obtained in this process (a mixture of gypsum and aqueous iron oxide) is exposed to air, under the influence of which it turns greenish to yellow.
By roasting yellow marshes, orangish, red, brown and purple marshes are obtained.
The color of these mars depends on the roasting temperature. A weak temperature gives orangish mars, a higher temperature gives darker mars.
Marls are much more transparent than ochres, in terms of durability they are not inferior to them.
Durable in all techniques and combinations. In oil they dry slowly. They need 50 to 60% oil for blending.
Recognition:
Marls composed of iron oxide and clay dissolve completely when heated in dilute hydrochloric and nitric acids.

English red, caput mortuum, Pompeian red

The better grades of these reds consist of pure anhydrous iron oxide, obtained artificially by various means: firing iron kyporoc and other iron compounds in their pure state in mixtures with salt or saltpeter.
These dyes have a high and medium specific gravity. They are durable, used in all techniques. Covering power good, oil needs 40 to 50%. They dry well.
Recognition:
In dilute hydrochloric acid they dissolve when heated, forming a yellow solution. In concentrated HCl they dissolve without heating. Spat and clay precipitate.