cupel cracking

R

rookieminer

New Member
#1
I'm very new to fire assaying, I've only done 6 so far. In all but one the cupel has cracked. I've read that when this happens that you will lose some of the pm's, is this correct?
I figured they were cracking because of moisture, but I leave them on the furnace during the fireing of the crucible, and put them in the furnace for about ten minutes before adding the lead, but still they crack. I'm cupeling between 1500f and 1750f. It's taking about an hour and fifteen minutes, sometimes a little less.
Could anyone give me a reason for my troubles? I appreciate any help, and don't offend easilly so don't hold back telling me if I'm doing it all wrong.

Thanks for any help

rookieminer
 
fireguy

fireguy

Supermoderator
#2
rookieminer said:
I'm very new to fire assaying, I've only done 6 so far. In all but one the cupel has cracked. I've read that when this happens that you will lose some of the pm's, is this correct?
I figured they were cracking because of moisture, but I leave them on the furnace during the fireing of the crucible, and put them in the furnace for about ten minutes before adding the lead, but still they crack. I'm cupeling between 1500f and 1750f. It's taking about an hour and fifteen minutes, sometimes a little less.
Could anyone give me a reason for my troubles? I appreciate any help, and don't offend easilly so don't hold back telling me if I'm doing it all wrong.

Thanks for any help

rookieminer
Hi Rookie:
1500-1750F is way to much variation (too cold and too hot). Find the best temperature for you (which will depend on the type and brand of cupel) and then keep the temperature as consistent as you can- start with 1650F. 1.25 hours sounds like a long time to cupel. How much do your lead buttons weigh? The typical cupellation, with a 1.5" cupel and 30 grams of lead, should take about 40-45 minutes. If it is taking longer than this you are probably not getting enough air (oxygen) in the furnace. Do you have a exhaust vent to move fumes out? An air inlet to let air in? What brand, and model of furnace?

With some more details we should be able to get things working better.
 
R

rookieminer

New Member
#3
Thanks for the reply fireguy,

I have a 9"x10"x6.5"Vcella furnace, and have been leaving the door cracked during cupelling.
I'm using 1.5" Magnesite cupels, and the lead buttons have weighed within 1 gram of 30 grams +or-.

I hope that helps,

rookieminer
 
R

RGJohn

New Member
#4
I have a few ideas.
One is, do your cupels come in a blister pack type of thing? It's possible for cupels to retain moisture quite a long time and you might want to take them out and dry them slowly for an extended period. Like days, not hours.
Regarding the question of losing values, yes the possibility is very real. How large is the crack? The bead may or may not dribble into it as the surface tension is so high. But it could happen. Pulverise and assay the cracked ones and you will know.
As for temp. I'd say run it hot until the buttons are clearly flowing litharge over their surfaces, then open that door and let all the air in you can. But you must maintain that good flowing surface temp. I'd say 1050 C.
If you ever decide to make your own cupels ( and due to such problems as you describe it has often been done), incorporate some molasses into your binder. No more cracks.
 
fireguy

fireguy

Supermoderator
#5
rookieminer said:
Thanks for the reply fireguy,

I have a 9"x10"x6.5"Vcella furnace, and have been leaving the door cracked during cupelling.
I'm using 1.5" Magnesite cupels, and the lead buttons have weighed within 1 gram of 30 grams +or-.

I hope that helps,

rookieminer
1) Be sure and preheat the cupels up to full temperature before adding the lead button.
2) Start at 1650F. Add the button to the preheated cupel and close the door for 10 minutes. You don't want to open the door until the cupels start "driving"- when they are "driving" they will turn bright red and become fluid.
3) Cracking the door is OK, but is not ideal. It is better to drill a hole in the roof of the furnace to let the fumes out. You can also drill a hole in the door to let air in. You may want to put an adjustable gate over the hole in the door so you can regulate the air flow. Generally, you want to move as much air as possible while maintaining temperature. If you don't want to drill the holes cracking the door will be OK, but you don't get very consistent airflow in the chamber. Lots of air flow close to the door, not so much further to the back.
 
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