bone ash cupel and magnesite cupel problems

[Jason]

Hello, I’m a Chinese chemist. The productions we used include bone ash cupel and magnesite cupel which were made by ourselves.
We have some problems here. When the temperature gets to 1050 degrees Celsius, the bone ash cupel will crack and fall off the bottom. If we place the magnesite cupel for a long time, they will automatically crack. Have you got such problems, can tell me the reasons?
Jason

 

[fireguy]

Hello, I’m a Chinese chemist. The productions we used include bone ash cupel and magnesite cupel which were made by ourselves.
We have some problems here. When the temperature gets to 1050 degrees Celsius, the bone ash cupel will crack and fall off the bottom. If we place the magnesite cupel for a long time, they will automatically crack. Have you got such problems, can tell me the reasons?
Jason

Hi Jason:
What binder are you using for the bone ash and magnesia (cupels are not made of magnesite- although many people mistakenly call them magnesite- including me sometimes )?

How long are you curing (drying) the cupels after you make them?
Are you making them in a press? If so, what pressure are you using?
What is the size distribution of the material?
Are you preheating the cupels up to full temperature before dropping the lead button onto them?

All of these factors will effect the process and can cause the problems you describe. You should be able to use a good quality magnesia cupel for all your analysis, and they are quite inexpensive. You might be better off purchasing the cupels:

http://www.lmine.com/category/magnesite_cupels.html