TECHNO TIP UNDERSTANDING CERAMIC STAINS Stuns are fired blends of metal and ce- Tamic oxides that have been reground iio afine powder. Stains containing olhenwise foxne oxides can be employed without significant dangers. This 1s the first aspect of something that stains have that colour- ing oxides dan’): stability, A second as- pectot stability is thutstains produce much more consistent ind repeatable colour than WSLNeG raw oxide colours. SIaIns ane most popular al lower tempera tures Where colours lend to be bnahter, However most stains can be used nght up to high lire. Premixed low fire glazes are typically made by blending stains and com- mercial fits and other than following fir- ing instructions, users of these products give litte thought to the technical challenges that Were Overcome to produce them. This is a third key advantage of stains: the ally lo Greet a specie eolour. Many colours like reds are difficull oacheve and beyand the abilities of end users. While mamy stains are slandant and their compo- slitan is well known, oth- ers are proprietary, Stain companies don’ trelease the exact makeup of stains but they &o tell us the system. For example, a green stain might contain chrome, co- balt and silica and be Ja- belled CrCoSi. Although silica is nota colorant itself, 111s included to create a stable silicate crystal structure with the other twa, Stain companies often supply multiple products to produce a given colour using different oxide systems (ic. chrome-tin pink, manganese-alumina pink). Various factors like the base glare chemistry, col- our shade, temperature, and end use deter- mine the system you should choose. Racw- ing how to calculate the oxide makeup ol a glaze or dealing with @ stain company thal provides good service are key factors to beng able to troubleshoot colour prob- lems with stains. February 2000 Tony Mamsen Thus stains do pat come with a unondl- honal eofour guanimlee. The shade pro- duced depends on many factors including the host glaze chemistry, on/over! unierglaze use, glaze thickness, amount of opacifier, firing temperature, etc. Achieving colours with staims 15 certainly easicr, but it is fot o no-brainer. Ceruun systems are quite flexible and produce colour in many kinds of glaves (ie. cobalt silicate), Other systems either require that ceriun oxides be present in Lhe host glaxe In TUMIMUM wMmGunts of others pot be present at all. The symbiosis of hast glaze chemistry and stain, for example, can be demonstrated with chrome-lin stains, They Will notdevelop colour if zine is presentor a: TULIA MATRA = majolica pottery from ber exhibition Sean mo Taste March (960 ai the Gallery of BC Ceramics if there is inadequate calcium. [t ts com- mon to hear people sivy (hal their pink stain burned out, but generally the stain is being used in an incompatible glaze base, An- other interesting demonstration of these factors is the colour chart of a typical stain company. These charts show the stain usec inane ar more [nitled base glazes that are sclected to be compatible with as many colours as possible, Certain samples will also have whded opaci{ier and zine, for example. Admittedly, stains can produce homoge- negus colour which can be less interesting than the variegated and speckled colour Potters Guild of British Colurbia elfects thatcan be achieved Wilh naw mctal oxides like iron, cobalt and copper, But for manufactunng, tt cannot be ignored thal stains are far mare consistent and reliable to ise, Potters love to pint stains over and under glazes to decorate ware. Majolica ware is a good example. However, keep in mind thal stains vary in their willingness to allow the underglaze to envelope the cal- our particles. Some tend to crystallize the surface if used overgluve. Likewise, il slains anc used underglaze they vary in their willingness to allow the overplave to penetrate through to fonm and interface with the body. Stains don” suspend well in Waler to create a paintable material either. Consequently i118 necessary to mix them with a compat ible and puintable glaze me- dium for daing brushwork. Stains exist either in the eon lext of the huge industrial ceramic industry or in the hobby, pottery, and ceramics markets. Large industnes ci- ther have in-house technical people or contnact comsull- anis. Small users do not have this luxury. They should know thal certain stun com- panies, (i.c. Mason) have de- veloped excellentrepatatons for dealing with smaller vol- umes and providing support. A fired glaze can leach heavy metals whether these metals are sourced [rom a stain or from raw metal oxides. You should have your glares tested for leaching if you are making Functional ware having stained food surfaces. There are many factors that determine if a glaze is leachable. For more information see the techne ip column in the last newsletter or visit http:/! digttalfire.com/education/glaze! leaching. htm Yeie fect Hp iy penerousy sponsored by Plainsman Clay Cimiled tn Medicine Hat, If you have a rechrical quecniov please cern i he the Guild affiee arenion the edinar.