Importance of Lead White in History
Lead white is the most important white pigment used in painting throughout history. It was known to the ancient Egyptians, Greeks, and Romans and commonly used in the preparation of ointments, plasters, and cosmetics. It was first identified in the literature as a pigment by Pliny, who mentions it, among other colors, as used by the ancients to paint ships.
The density of lead white and its opacity is critical to painters. If lead white were not dense and opaque, it would be necessary to put it on thickly where, since it is both dense and opaque, a thin coat of it will serve. The power it gives when making strokes of light as incisive as strokes of dark paint is fundamental to the painting technique.
Composition and Importance in Oil Paint
Lead white oil paint is made by grinding basic lead carbonate (the chemical name for lead white pigment) in vegetable drying oil. Normal lead carbonate (PbCO3) or other lead carbonate compounds have often been mistakenly identified as basic lead carbonate, which has the chemical formula 2PbCO3•Pb(OH)2. The different varieties of lead carbonate have not been reported in paintings except as impurities. Basic lead carbonate contains 25 to 30 percent lead hydroxide, distinguishing it from normal lead carbonate. It is this element that gives added opacity to lead white. It also gives it the qualities in oil paint often sought after by artists.
Lead white also serves to preserve the flexibility of oil paint, extending the life of the paint film. In a posting on the AMIEN forum, Dr. Jaap Boon, University of Amsterdam and AMOLF and formerly head of the MolArt Project, offered these crucial insights into the role of lead white in oil paint film:
“The role of lead white is multiple. It is a bulk drier that means homogenous drying over the paint film. It reacts with free acids that develop by oxidation of the oil. It links the acids groups of C9 diacids even when they are still acylglycerides thus stabilising the early film (proven by NMR work of Michiel Verhoeven in 2006). At a later stage when the biological ester bonds are gone, it further stabilises the diacids to form a network that is hard to break. And furthermore, it provides a filling material and surfaces for oil-derived compounds to dock.”
“No lead means that something else has to take over these roles. I don’t think that that magical material has been found yet simply because we only very recently have begun to understand the molecular structure of paints. It is my belief that the period 1950–2000 will bring us many defects in the oil paints that are now maturing. It is time that we develop a molecular level understanding on how to deal with oil paint defects that develop when the network can’t be stabilised with suitable metal cations.”
Why “Flake White” and “Cremnitz White” for Lead White
When you see a tube of oil paint labeled “flake white” or “Cremnitz white,” what pigment does it contain? Most artists’ material manufacturers call flake white—a mixture of basic lead carbonate and zinc oxide ground in vegetable oil (usually linseed or safflower). Cremnitz white, on the other hand, is typically pure basic lead carbonate ground in vegetable oil.
Lead white has been made since antiquity using variations of the process known as the “stack process” or “Old Dutch” method, whereby lead is exposed to acetic acid vapor in the presence of moisture and carbon dioxide; the latter is generally provided by fermenting matter (horse manure, waste grape skins, tanbark), which also provides a constant source of heat. In this process, the air supplies the oxygen, while the fermenting matter produces carbon dioxide and moisture. The acetic acid, in the form of vinegar, converts the lead, forming basic or tribasic lead acetate, which is afterward decomposed by the carbon dioxide to form basic lead carbonate. Stack process lead white and its related Krems process differs from methods used to manufacture basic lead carbonate today, based on an electrolytic process. The results of the Dutch and Krems methods are basic lead carbonate, but the size and shape of the pigment particle are quite different, giving these whites other properties in paint. A more detailed discussion of the stack process can be found in Stack Process White Lead—Historical Method of Manufacture.
The term flake white takes its name from the scales or flakes (see right) of basic lead carbonate that appears on corroded lead in the stacking process or the old Dutch method of making the pigment. The modern electrolytic process does not result in flakes of basic lead carbonate but rather the pigment deposits as a fine precipitate. The particle size and morphology (crystal structure and particle shape) of stack process lead white is a critical factor that makes this pigment different from modern lead white.
More About Stack Process White Lead
The name Cremnitz white is a variant spelling of Kremnitz, derived from the German name of the Slovak town Kremnica. This town was mistakenly associated with the commercial name Kremsweiss (German) or Krems white famous in the 18th and 19th centuries. Krems white was manufactured in Klangenfurt, Austria, but the name was probably derived from the galena (a lead ore) mines near the small town in lower Austria, Krems, now called Krems an der Donau. Krems white was deemed superior to any other lead white during the 19th century, and it is perhaps why artists’ materials manufacturers chose this name to designate their pure lead white oil paint.
Rublev Colours Lead White
Rublev Colours Lead White #2 is an opaque white that is soft, ropy, or “stringy.” The ropy consistency of Lead White #2, also called “long,” is a result of using walnut oil without additives. Long paint pulls with the brush and, for this reason, is often preferred by artists because it flows with your brushstroke enabling long lines. This effect can be seen in the image at the right. Lead White #2 is a brighter white than Rublev Colours Lead White because it is ground in filtered, pale walnut oil.
Because Rublev Colours Lead White #2 is ground with basic lead carbonate made according to modern practice, we chose to name it “Lead White” so as not to confuse artists with lead whites made according to the stack process or Krems process named “Flake White” and “Krems White,” respectively.
The image above is a “drawdown” of Rublev Colours Lead White #2 oil paint. The left drawdown is the color at full strength at three mils (0.003 inches) thickness over a black and white chart. The right drawdown is one part of the color mixed with three parts of bone black. The bottom edges of the drawdowns were scraped to show the color of the undertone.
Additives are used in modern oil paint to help prevent the pigment and oil from separating during storage and as an aid in grinding the paint. Rublev Colours Lead White #2 contains basic lead carbonate (made according to modern processes) without additives or fillers to alter the pigment characteristics in oil. As a result, you get a higher pigment volume concentration (PVC) than other brands of lead white (flake white)—as high as 45%. This means most brands of flake white do not weigh nearly as much as a 50 ml tube of Rublev Colours Lead White #2. Yet, Rublev Colours Lead White is not stiff and mixes well with all other oil colors.
Additives used by all other manufacturers include beeswax, hydrogenated castor oil, and aluminum or magnesium stearate. These ingredients amount from one to two percent of the pigment weight in the formula. Although this is a comparatively small amount, additives change the behavior of the pigment in oil because they increase the oil’s viscosity, giving the paint a buttery, short consistency.
Rublev Colours Artists’ Oils are traditional oil paints because they are made according to historical methods, such as those in the 19th century, when modern additives were not used in artists’ paint.