Tin oxide is the traditional opacifier: added to a transparent glaze it turns it an opaque, soft white. It is also one of the most expensive materials on a supplier's price list, which is why zirconium-based opacifiers exist and why most modern white glazes use them instead.
How opacifying works
Tin oxide does not dissolve into the glaze melt. It stays as fine particles suspended in the glass, scattering light so the glaze reads white and opaque rather than clear. The more you add the more opaque it becomes, up to a point beyond which it starts to matt the surface as well.
Why tin whites look different
A tin-opacified white has a softness and warmth that potters describe as creamy, and it interacts with colourants beneath and within it in ways zirconium does not. The classic case is with chrome, where tin can produce pink rather than the expected green, an effect used deliberately in some traditions and encountered accidentally by everyone else.
The price, and the alternative
Tin is expensive by the kilogram and is used at five to eight percent, which makes it one of the few materials where the cost is noticeable in a bucket of glaze. Zirconium silicate opacifiers give a harder, colder white at a fraction of the price and are used at higher percentages. Most commercial white glazes use zirconium; potters who prefer the tin white pay for it knowingly.
Buying and testing
Because of the cost, buy small and test a line blend at four, six and eight percent on your own base before mixing a bucket. Fired tiles side by side show the difference between tin and zirconium whites clearly, and the choice is genuinely a matter of preference rather than of quality.
Questions people ask about tin oxide
What does tin oxide do?
It opacifies a glaze, turning a transparent base into an opaque white by staying suspended as fine particles that scatter light.
Is tin oxide worth the price?
For the particular soft white it gives, some potters think so. Zirconium opacifiers give a harder, colder white for far less.
Why did my tin glaze go pink?
Tin with chrome present gives pink rather than green. It is a well-known interaction, used deliberately in some traditions.