|Monthly Tech-Tip |
|Frit Softening Point||817C (From The Oxide Handbook)|
-Bismuth makes it possible to push the melting temperature of boron frits down by many hundreds of degrees. Using sufficient Bi2O3, it is possible to make frits that melt at temperatures as low as 750F (400C)! Boron frits with additions of less than 5% bismuth can melt below 1200F (although some Li2O or F is also needed). With sufficient Li2O to help, even high percentages of ZrO2 can even be tolerated and still achieve near 1100F melting.
-Bismuth is a secret to making very low temperature frits and colors, conductive glazes, enamels for metal and on-glaze colors. It is also used in glass frits, these melt at low enough temperatures to make bonding (gluing together) of diverse materials possible.
-During the 1990's, industry has been under much pressure to discontinue the use of lead compounds. Like lead, bismuth ions are highly deformable and thus a strong flux. Bismuth is a very effective substitute for PbO in frits, providing the same high gloss, flow, 'healing' and 'bubble clearance' characteristics, refractive index, surface tension, viscosity, and resistance to 'aggressive' dishwasher detergents. Bismuth melts much lower than lead and thus glazes can be even more fluid. One difference for some to note is that its response to cadmium colorants is different (although using inclusion stains should not be a problem).
-Although we do not have an expansion number for Bi2O3, many people simply use the one for lead.
-Bismuth has been used instead of lead oxide in amounts up to 50% in optical glasses to improve durability and increase the specific gravities and refractive indexes. Arsenic is often used with it to prevent a tendency toward grey coloration.
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).
Using bismuth it is possible to create very low melting glasses (or glazes). But a very high percentage is required. And bismuth is expensive.
Fusion Frit FZ-915
|Oxides||PbO - Lead Oxide|
|Oxides||F - Fluorine|
Bismuth Oxide on Wikipedia
A Bismuth Glass the melts at 400C
|By Tony Hansen|
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