The Shop > Metal Stuff
Plaster of Paris, Investment, etc.
vtsteam:
True Andrew, but I want to know what the basic chemical reaction temperature is, even if real world variations in plaster are a factor. I can extrapolate out for that, but I want a base understanding of the chemical process.
vtsteam:
Okay here's what I mean -- excuse the tech talk, but this is an example of how specific this kind of thing can be -- unfortunately it's not for plaster of Paris:
--- Quote ---Calcination reactions usually take place at or above the thermal decomposition temperature (for decomposition and volatilization reactions) or the transition temperature (for phase transitions). This temperature is usually defined as the temperature at which the standard Gibbs free energy for a particular calcination reaction is equal to zero. For example, in limestone calcination, a decomposition process, the chemical reaction is
CaCO3 → CaO + CO2(g)
The standard Gibbs free energy of reaction is approximated as ΔG°r = 177,100 − 158 T (J/mol).[3] The standard free energy of reaction is zero in this case when the temperature, T, is equal to 1121 K, or 848 °C.
Examples of chemical decomposition reactions common in calcination processes, and their respective thermal decomposition temperatures include:
CaCO3 → CaO + CO2; 848 °C :
--- End quote ---
vtsteam:
Okay, so elsewhere in Wikipedia it says:
--- Quote ---Gypsum plaster, or the plaster of Paris, is produced by heating gypsum to about 300 °F (150 °C):[2]
CaSO4·2H2O + heat → CaSO4·0.5H2O + 1.5H2O (released as steam).
When the dry plaster powder is mixed with water, it re-forms into gypsum. The setting of unmodified plaster starts about 10 minutes after mixing and is complete in about 45 minutes; but not fully set for 72 hours.[3] If plaster or gypsum is heated above 392°F (200°C), anhydrite is formed, which will also re-form as gypsum if mixed with water.[4]
--- End quote ---
A bit more approximate (about 300C) and the source cited is a dead link, so not entirely sure. And a new fact comes to light "anhydrite" which means no water? Not even the 0.5 people usually cite in the reaction. So there are two different calcining points? Unclear yet.
mattinker:
--- Quote from: vtsteam on August 04, 2014, 07:55:29 AM ---Okay, so elsewhere in Wikipedia it says:
--- Quote ---Gypsum plaster, or the plaster of Paris, is produced by heating gypsum to about 300 °F (150 °C):[2]
CaSO4·2H2O + heat → CaSO4·0.5H2O + 1.5H2O (released as steam).
When the dry plaster powder is mixed with water, it re-forms into gypsum. The setting of unmodified plaster starts about 10 minutes after mixing and is complete in about 45 minutes; but not fully set for 72 hours.[3] If plaster or gypsum is heated above 392°F (200°C), anhydrite is formed, which will also re-form as gypsum if mixed with water.[4]
--- End quote ---
A bit more approximate (about 300C) and the source cited is a dead link, so not entirely sure. And a new fact comes to light "anhydrite" which means no water? Not even the 0.5 people usually cite in the reaction. So there are two different calcining points? Unclear yet.
--- End quote ---
I'm not sure I understand what you don't understand! I have found several sources (French plaster manufacturers) that say that the three out of four molecules of water are driven off at above 140 to 150°C, from what I can see, the Anhydrite CaSO4 is over 200°C The only thing I haven't found is what happens to plaster when heated over 200°C, does it too become Anhydrite?
I remember being told 'but I don't remember who by!) that making plaster was an irreversible process, I wonder whether the initial heat cycle prevents the loss of the remaining molecule of water.
Regards, Matthew
vtsteam:
It is not a matter of "understanding" Matt.
Water boils between 200 and 250F, but even my 8 year old can get more specific than that.
And in fact, that specificity is important to an understanding of much in science and history that follows from it.
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