From LouisKatz

Main: DynamicsOfMelting

Article on the dynamics of melting and solidification.

In ceramics we think of glazes as melting. Glazes do melt and denying this would be foolish. But in complex systems like glazes thinking of melting is not as useful as thinking of solids going into solution. Most of our materials, at least by weight do not melt at temperature that we can attain in our kilns. They dissolve into already melted materials.

I like to think of crystalline solids, like our ceramic materials as buildings built out of girders. Each sort of building has girders of different sizes and not all the joints are arranged for 6 girders to join together. As the materials heat up the buiding begins to shake but when shaking mildly it can still hold to.......... When we heat our clay some materials begin to melt at quite low temperatures below normal bisque. Usually these materials are small in quantity. Once they are melted other materials are slowly dissolved into them. This greater volume of liquid acts as a pool into which other materials dissolve. When a solution will dissolve no more of a material at a given temperature it is said to be saturated. Generally as you raise the temperature you raise the amount of a material that can be dissolved in a given liquid. What happens when you are dissolve say a chunk of say sugar into a light syrup is that around the lump of sugar you create an amount of liquid that is closer to being saturated. Until the more saturated solution disperses the dissolving slows down. Dispertion can be aided by stirring, but by itself it relies on convection and Brownian motion. Some materials dissolve quite slow. One reason for this is that they make the solution more viscous slowing dispersion.

Equilibirum in relation to glaze melting is the condition where if you hold the temperature for any amount of time, no more material will dissolve or melt. In our firings we never even come close to equilibrium At 2350 F there is only so much silica that you can dissolve into a calcia alumina silica glass even if held for days at that temperature. Because we only fire with short holds we do not even come close to these limits. Our lack of holding to equilibrium makes cones a good way to determine the status of our ware. Cones melt as glazes do, really they are just glazes. A cone 5 makes apassable cone 9 or ten glaze. A cone 7 should make a good cone 12 glaze and so on.

Things change form as they melt. Sodium Carbonate in glazes gives off carbon dioxide. It melts into a glass so it really does not exist as descrete NA2O? but those are the elements left after melting. The glass like other liquids has the ability to dissolve some of the gas given off, but most of it is expelled as bubbles of CO2?. Other materials

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