End point of the cupellation

C

crauther

New Member
#1
Is there any tell tale sign for the end point of the cupellation process? We normally run cupellation for nearly 14 to 19 minutes, depending on the previous result those time could change, by comparing result from the check gold sample, two check gold and ten samples at each section, and temperature at 1050 degree Celsius. After the cupellation, switched off the furnace, allow them to reach 870 degree Celcius. But at certain time, three or four times in each day, we never let the temperature to reach 870 degree, and download the cupel at 1000 degree celsius. Will this premature download effect the result?
 
fireguy

fireguy

Supermoderator
#2
Is there any tell tale sign for the end point of the cupellation process? We normally run cupellation for nearly 14 to 19 minutes, depending on the previous result those time could change, by comparing result from the check gold sample, two check gold and ten samples at each section, and temperature at 1050 degree Celsius. After the cupellation, switched off the furnace, allow them to reach 870 degree Celcius. But at certain time, three or four times in each day, we never let the temperature to reach 870 degree, and download the cupel at 1000 degree celsius. Will this premature download effect the result?
I think you are referring to two different process here.

The actual end of the cupellation itself is indicated by the "blick".

Once the lead starts driving and oxidizing it turns a bright red color:
19242a.jpg

When completed, the color will turn dark, and then there will be a bright flash as the last residual lead it oxidized. This flash is called the blick.

For typical routine ore fire assays you really don't need to do any special cooling after this stage. The cupels are usually pulled straight out of the furnace and allowed to cool at room temperature with no problems.

Large samples, like bullion analysis, especially if they have a high silver content need to be cooled more carefully. If large silver based beads are pulled out of the furnace and allowed to cool too fast the silver will sprout. When the hot bullion cornet hit the cold ambient air it will rapidly oxidize and form a protrusion. This can cause high losses and should be avoided. Typically for these type of samples you would turn the furnace off and open the door to allow it to cool more slowly (this may be what you are doing).
 
C

crauther

New Member
#3
I think you are referring to two different process here.

The actual end of the cupellation itself is indicated by the "blick".

Once the lead starts driving and oxidizing it turns a bright red color:
19242a.jpg

When completed, the color will turn dark, and then there will be a bright flash as the last residual lead it oxidized. This flash is called the blick.

For typical routine ore fire assays you really don't need to do any special cooling after this stage. The cupels are usually pulled straight out of the furnace and allowed to cool at room temperature with no problems.

Large samples, like bullion analysis, especially if they have a high silver content need to be cooled more carefully. If large silver based beads are pulled out of the furnace and allowed to cool too fast the silver will sprout. When the hot bullion cornet hit the cold ambient air it will rapidly oxidize and form a protrusion. This can cause high losses and should be avoided. Typically for these type of samples you would turn the furnace off and open the door to allow it to cool more slowly (this may be what you are doing).
A lot of thanks. We are adding 2.3 times silver to the bullion sample. Last month, during a tight schedule, we were forced to take out the cupel, just after the allowed time for cupellation, 19 min, and we found the "blick" happened outside the furnace. Before the blick happened, we noticed a molten substance shifting on the top of the bead, revolving like an eye. No sprouting had taken place, the beads are healthy but there could see "dimple" appearance on each and every dore bead. Meanwhile we got a satisfactory result for that test. What am looking for, is it okay the" blick" to happen outside the furnace , while covering it with red hot cupel, as in the earlier example
 
fireguy

fireguy

Supermoderator
#4
It may not be TERRIBLE for the blick to happen outside the furnace, but I think it would be better on the inside so the blick happens at the elevated temperature. But, I have seen the blick outside the furnace with no obvious problems.

Are you running corrected bullion against a proof? If you are worried about ultimate accuracy you should do this. There is a good write up on the proper corrected bullion method in the Volume 2 Methods Manual (this was ultimately adopted by ASTM for their method):
www.lmine.com/product/9051x.html
 
C

crauther

New Member
#5
It may not be TERRIBLE for the blick to happen outside the furnace, but I think it would be better on the inside so the blick happens at the elevated temperature. But, I have seen the blick outside the furnace with no obvious problems.

Are you running corrected bullion against a proof? If you are worried about ultimate accuracy you should do this. There is a good write up on the proper corrected bullion method in the Volume 2 Methods Manual (this was ultimately adopted by ASTM for their method):
www.lmine.com/product/9051x.html
Thanks.........
 
W

wotavidone

New Member
#6
A lot of thanks. We are adding 2.3 times silver to the bullion sample. Last month, during a tight schedule, we were forced to take out the cupel, just after the allowed time for cupellation, 19 min, and we found the "blick" happened outside the furnace. Before the blick happened, we noticed a molten substance shifting on the top of the bead, revolving like an eye. No sprouting had taken place, the beads are healthy but there could see "dimple" appearance on each and every dore bead. Meanwhile we got a satisfactory result for that test. What am looking for, is it okay the" blick" to happen outside the furnace , while covering it with red hot cupel, as in the earlier example
The revolving substance on the surface of the prill is the very last of the litharge. It's called the "play of colours". Prills are less prone to sprouting if they are allowed to solidify, and blick, inside the furnace where it is nice and warm. However, if you saw the blick, it is an indication that the cupellation has proceeded as it should. Prills have to be fairly pure to get a decent blick.
 
W

wotavidone

New Member
#7
I think you are referring to two different process here.

When completed, the color will turn dark, and then there will be a bright flash as the last residual lead it oxidized.
Here's how it was explained to me nearly 40 years ago. When cupelling silver or gold, you actually run the muffle cooler than the melting point of the pure metal. The lead oxidising keeps the alloy hot enough, as the lead is removed and the purity increases, to stop the whole melt freezing.
Then right at the end you see the "play of colours" as the very last of the molten litharge imparts an iridescent sheen to the molten ball of silver or gold. For a nice clean low-loss prill, you run cool until this point, then bump the temperature back up to make sure the process is hot enough to continue right to the end.
Then, with no more exothermic reaction happening, you are left with this strange phenomenon of having a ball of molten metal, silver or gold or dore, that is actually colder than its melting point...
The old timers called this "sur-fusion".
At this point, the outer shell electrons of the silver and gold atoms are in an elevated state. They fall back to the ground state, and in the process release the energy that was holding them up. This energy is emitted as visible light - white/yellow for silver, greenish white for gold.
 
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