Tin oxide, also known as Stannic Oxide, is a white or off-white powder produced by oxidizing molten high grade tin metal. It is typically quite pure, some manufacturers have grades up to 99.999% purity.
Tin oxide has long been used to opacify glazes in oxidation (make transparents opaque) at all temperatures. Many potters now use zircon based opacifiers instead. Thus, any discussion about the use of tin oxide as an opacifier ends up comparing it with zircon products:
-Twice as much zircon is required to produce the same level of opacity.
-Like zircon, tin melts at very high temperatures and thus does not go into solution in typical glaze melts.
-Zircon will stiffen the glaze melt more than tin.
-Zircon will likely produce a harder glaze surface.
-Zircon will reduce the thermal expansion of the glaze more than tin.
-The quality of the white colour is different (tin tends to be more of a blue white, zircon a yellowish white).
-Zircon tends to have less of an effect on the development of metal oxide colours (e.g. tin reacts with chrome to make pink).
-If gloss is an issue, silica might have to be reduced to compensate for the silica introduced by a zirconium silicate opacifier being substituted for tin.
-While there are other products that produce varying degrees of opacity, none are as neutral and non-reactive as tin and zircon. Other opacifiers also tend to variegate the glaze.
-Tin does not normally opacify in reduction firings.
Tin is also a player in the development of ceramic colours, for example chrome tin pinks and maroons. Tin with iron in oxidation makes a warmer shade of brown than zirconium does.
Tin oxide is also a variegator. For example, tin can react with titanium and rutile to completely transform the colour and character of a glaze. Although tin is expensive, very little is required to produce stunning effects in many coloured glazes.
Glaze Opacifier - White
As little as 4-7% can produce brilliant white, although it is more typical to use 8-10% for full opacity. However, be aware that even tiny amounts of chrome in the kiln will volatilize and combine with the tin to produce pink shades.
Glaze Variegation - Tin/Iron Effects
Tin reacts with iron in fluid glazes to produce variegated surfaces. A good example is the Albany Slip 85, Tin 4, Lithium 11 glaze for cone 6.