|Monthly Tech-Tip |
|Co-efficient of Linear Expansion||0.020|
|Frit Softening Point||1625C (From The Oxide Handbook)|
More correctly called Tin Dioxide
-The fully oxidized state of tin metal. It is a very white powder of low density. Although tin metal melts at a very low temperature, the oxide form is quite refractory.
-Tin oxide has been used primarily as an opacifier in amounts of 3-15% in all types of glazes for many centuries. The amount required varies according to the glaze composition and temperature. The mechanism of the opacity depends on the white tin particles being in suspension in the molten glass. At higher temperatures, these particles will start to dissolve and opacity will begin to be compromised. In a cone 10 reduction firing, for example, 4% tin in a transparent will have no opacifying effect.
-Like zirconium oxide, larger amounts of tin in lower temperature glazes have a refractory effect, stiffening the melt and increasing the incidence of pinholing and crawling.
-Tin white is considered a softer white than that produced by the very popular and much cheaper zirconium opacifiers.
-Tin forms pink colors with chrome oxide (if the chemistry of the host glaze is right). The reaction is very sensitive, and even minute amounts of chrome can impart a pinkish hue to a tin-opacified glaze. This sensitivity can be a problem if volatile chromium is flashing in the kiln atmosphere from other glazes, the white color of tin opacified glazes anywhere in the kiln can be affected.
-Other opacifiers include zirconium oxide (gives a harsher glassy white), calcium phosphate (problems with off-coloring to greys), cerium oxide (restricted to low temperatures), antimony (dissolves in some glazes and gives yellows with lead), and titanium dioxide (discolors if any iron oxide is present and crystallizes easily).
All common traditional ceramic base glazes are made from only a dozen elements (plus oxygen). Materials decompose when glazes melt, sourcing these elements in oxide form. The kiln builds the glaze from these, it does not care what material sources what oxide (assuming, of course, that all materials do melt or dissolve completely into the melt to release those oxides). Each of these oxides contributes specific properties to the glass. So, you can look at a formula and make a good prediction of the properties of the fired glaze. And know what specific oxide to increase or decrease to move a property in a given direction (e.g. melting behavior, hardness, durability, thermal expansion, color, gloss, crystallization). And know about how they interact (affecting each other). This is powerful. And it is simpler than looking at glazes as recipes of hundreds of different materials (each sources multiple oxides so adjusting it affects multiple properties).
Formulating a Clear Glaze Compatible with Chrome-Tin Stains
In ceramics color is often a matter of chemistry, that is, the host glaze must be compatible and have a sympathetic chemistry for the stain being added. Chrome-tin stains are a classic example.
Tin Oxide at Wikipedia
|Glaze Color||Chrome and tin are the most well known way to produce pink. For example, 7.5% tin and 0.5 chrome oxide will produce pink. Many Cr-Sn stains are available to make many shades on pink. However this mechanism requires that the glaze chemistry be right (e.g. no zinc, boron not excessive) for slow firings (in industry firing is typically so fast that the stain does not get an opportunity to react with the zinc).|
|Glaze Opacifier||Tin is an effective opacifier to transform transparent glazes to white. The quality of color tends to be a 'soft-bluish white' compared to harsher effects with other oxides.|
|By Tony Hansen|
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