Venetian Red Pigment: History and Modern Applications

The natural form of Venetian red pigment consists of iron oxide, gypsum, terra alba, and other materials associated with the mineral hematite. The exact constituents of this pigment vary from region to region, but they essentially remain the same. The pigments are carefully ground and processed to create a smooth, consistent color that is easy to use.

Before the 20th century, synthetic Venetian Red was made using the copperas process, which involves direct roasting of iron sulfate at high temperatures with stoichiometric quantities of sulfur oxides separated off. [18] Iron oxides of the copperas process generate by-products that are extremely problematic, as their corrosiveness and toxicity require intensive waste gas cleaning. The iron oxide pigments formed during the roasting process still contain soluble metal sulfates that need to be removed by washing before the pigment can be dried. The wastewater contains iron sulfate, together with other soluble heavy metal sulfates, and requires very complex cleaning or needs to be disposed of.

The copperas process begins with the precipitation of iron oxide from iron(II) sulfate or chloride, with the precipitation accomplished using lye such as sodium hydroxide. Given the high temperature required for the precipitation process, it is very energy-intensive compared to other processes and also requires an immense amount of water.

Since the 20th century, three other methods are used to make synthetic iron oxides—Laux, calcination, and Penniman processes. In the Laux Process, iron oxide is obtained as a by-product of the aniline process. The main pigment obtained by this process is black iron oxide, and also some amount of yellow iron oxide. These pigments are sold directly or used to prepare red iron oxide by the calcination process. The calcination process starts with black iron oxide oxidizing to yellow iron oxide and then dehydrating it to red iron oxide pigments. High temperatures are required for this process. Precisely controlling the process parameters also allows the production of brown pigments. The Penniman process reacts metallic iron with oxygen from air and acids as catalysts to obtain yellow iron oxides. Recent innovations in the process have been developed to also give red and black iron oxide pigments.

To make synthetic Venetian Red, red iron oxide produced using one of the above processes is mixed with chalk in proportions of one to four or less, depending upon the desired shade of red. Other extender pigments provide different density and hue properties that may be desired by the manufacturer. The resulting hues and tints with white will be different from one sample to the next, so that the hue is not standardized. 

Read more: Natural Pigments Venetian Red