Seeing red: Paul Lewing has some tips for cadmium colours in China paint.
CADMIUM COLOURS ARE, as every china painter knows, the trickiest group of colours. They are notoriously hard to mix with other colours or fluxes, and tend to disappear entirely if conditions are not exactly right. When they disappear, they usually take any other colour with them, with no possibility of getting the colour back in subsequent firings. Several factors in the composition of the colourant, the flux environment and the firing complicate their successful use. However, when they work, they are the brightest of the reds and oranges. The behaviour of cadmium colours in low-fire glazes corresponds fairly well to their characteristics in china paints, although the range of shades available is much more limited.
Cadmium was first identified in 1817 as a byproduct of zinc refining, and its name comes from the Latin word for zinc ore, cadmia. However, it is rare, and there was no production of the oxide to speak of until 1840. It was not used in china paints until much later, well into the middle of the 20th century. In 1930 the Roessler & Hasslacher Chemical Company listed in its catalogue both selenium and cadmium (as metal, sulphide orange and yellow, and yellow), although it is unclear (and probably unlikely) whether any of its 340 overglaze hues were cadmium colours.
It is a misnomer to refer to these shades as cadmium colours, since they all contain some selenium as well. Some colours in the group actually contain more selenium than cadmium, and the two are quite different from each other. The actual solid suspension crystal that produces the colour is a mixture of cadmium sulfide (CdS) and cadmium selenate (CdSe), known as cadmium sulfoselenate (CdS/CdSe). This is an unstable compound, and easily decomposed. Note that, unlike virtually all other ceramic colourants, neither of these is an oxide; cadmium oxide is white.
The range of colours produced by CdS/CdSe is determined partly by the proportions of the two crystals; the more CdS is present, the more yellow the resulting stain will be; the more selenium is present the redder the colour, with the extreme being maroon. The colour sold by some companies as 'mixing yellow', made to blend with the other shades in this group, contains the most CdS. However, other companies sell a colour billed as 'mixing yellow' that is not a cadmium colour at all, made to simply blend with all their other colours, particularly the iron reds.
The balancing act that produces these colours begins in the formulation of the stain. Cadmium needs an oxidising agent, such as a nitrate, to retain its yellow colour. Selenium needs the opposite: a reducing agent, usually metallic silicon, to retain its red. Too much of either agent will destroy one or the other of the colours. A little zinc in the mix will brighten the reds, and a little vanadium pentoxide is sometimes added to prevent discolouration.
The flux composition is also critical. If there is too much lead, the black compound lead selenide forms. Often a little cadmium oxide is added to the flux to stabilise the colour. However, bright cadmium china paints cannot be made without lead. One US company that produces lead-free china paints makes their only exception for their cadmium red, which they have manufactured for them in Mexico.
As stated above, a little too much oxidation or reduction in the firing will destroy one or the other colourant. This is the reason that cadmium/ selenium colours need a thicker application than other colours: it protects them from the atmosphere. It is usually recommended that the cadmium colours be fired to a slightly cooler temperature than most other colours of china paint, cone 017 or 018 as opposed to 015 or 016. However, I fire them to 016 all the time and they seem to work well. Cadmium colours like a fast firing, and they don't like to be crowded. I usually fire my tiles in mullite racks, and if I want really bright reds, I leave every other slot in the racks empty.
It will sometimes work to apply these colours over other fired colours, but another colour or a clear flux applied over a fired cadmium/selenium colour will usually destroy it, often taking the covering colour away as well. The fusion of the top coat with the underlying one destroys the precise flux balance necessary for bright reds and oranges. Many of the colouring oxides, such as iron, are reactive, and disrupt the delicate oxidation/reduction balance needed for bright colour.
A few brands of overglaze include some other colours designated as cadmium colours, particularly cadmium green, blue, black and white. These are not true cadmium colours, in the sense that the colourant is not CdS/CdSe. These colours simply contain some cadmium oxide in the flux mixture, and are made to blend with the cadmium reds and yellows.
Cadmium-inclusion stains, a recent breakthrough in ceramic colour at higher temperatures, are made by encapsulating cadmium in a zircon crystal. They are never used in china paints because they contain too little cadmium to produce a strong colour and too much zirconium to melt at that temperature. These stains also require a base glaze high in calcium oxide to produce their brightest colours, which makes them too refractory for use in china paints.
It should be noted that the cadmium colours are extremely hazardous to use. Cadmium and lead are the only two elements for which the US Food and Drug Administration, as well as the State of California, has established standards for leaching from dinnerware. The allowable limit on plates for lead is about six times the limit for cadmium.
Paul Lewing is a the artist from Seattle, Washington, and the author of China Paint & Overglaze, published by American Ceramic Society.
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|Date:||Jul 1, 2007|
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