4metals,
Excellent writeup. I have a few comments on the particular way I did things. I'm not criticizing. Both ways will work.
Equipment and supplies:
Hot plate - I found that an old electric skillet is excellent for parting and drying the gold. I assume that's what you meant. They are very controllable. If I used an actual hot plate, I would set the parting crucibles in a white, flat-bottomed CorningWare dish.
Pliers - See 15 below
Cupels - For silver assays, bone ash is best since you can see the feathers. For electronic assays, I prefer the composite cupels (bone ash + portland cement), since they are much more durable (and cheaper). Although about any size of 1 inch, or bigger, is good, I prefer the 1-3/4" size for electronic materials.
Coors ceramic annealing cups - Actually, annealing cups are clay and are made by such companies as DFC. I think what you mean are Coors porcelain crucibles. I prefer the high form ones that hold about 30 ml. I use them for parting, drying, and annealing.
Procedure:
8 - I usually numbered the crucibles, but not the cupels. It was too easy to keep track of them. At one place, we ran 36 cupels at a time in a beautiful DFC gas assay furnace. To keep track of the beads, I put them in a spot plate, on which I had numbered the depressions with a magic marker.
9 - I always preheated the cupels, at full temp., for about 20 minutes or more, before adding the lead. If you don't drive out all the moisture, the moisture will spit molten lead out of the cupel and you will have to start over. I also preferred to set the front row of cupels back about an inch from the front of the floor of the furnace. Otherwise, they can freeze up (turn black), since they can get too much air.
10 - 10 minutes seems like a lot of time, normally. Once the lead changes from black to a bright glow, they are ready to start driving. I would guess this normally takes 2 or 3 minutes. Rarely, some will stay black and won't open, no matter how long you leave them. You can try to open them by raising the temp and/or putting them in the back of the furnace. If this doesn't work, the last resort is to hold a splinter of wood near the top of the lead with tongs. The carbon will sometimes reduce the black lead oxide to lead metal. If this doesn't work, start over.
12 - Sometimes, if you have a lot of cupels, the ones in the back will take forever because they are not getting enough oxygen from simply cracking open the door. For this reason, I always drilled about a 3/8"-1/2" hole through the back of the furnace, centered and about 2" or 3" above the furnace floor. This allowed airflow over the top of the cupels.
14 - I don't like timing the cupellation. Some are done sooner than others, due to such things as their position in the furnace. Also, when cupelling the lead from the fusion of electronic parts, the weight of lead will vary. I like to remove each cupel from the furnace when it is finished. When the bead changes from a bright, rotating, glowing orb to a gray metallic bead, all the lead is out and it is finished. Silver sprouting can occur with a high percentage of silver or too high a temperature. I cupelled at about 1750-1800F. Gold losses can occur with sprouting. If I had trouble with sprouting, I covered the cupel carefully with an equally hot upside down cupel and then removed both together from the furnace. This doesn't work as well with bone ash, since it can easily crumble when handling it with tongs, especially when the cover cupel is used multiple times.
15 - I found that 6" electrician pliers work well for removing stuck beads from cupels. Use them vertically. Squeeze and twist. The slight squeeze will loosen any bone ash that is stuck to the bottom of the bead. It can then be easily brushed off.
16 - If you're only interested in gold, add silver to to the original sample. If you are interested in both gold and silver, the procedure is - wrap in lead and cupel - weigh and record the weight - wrap in lead with added silver and cupel - part - dry - anneal - weigh - calculate.
17-23 - I always flatten the bead using a very clean hammer and anvil before parting. I added about 15 ml of acid (Note: I only used .25 gm samples for karat gold and less acid was needed) to the porcelain crucibles and heated it to slight steam before adding the beads. I first used a 1/7, HNO3/distilled H2O mix. When I got no more fizzing, I decanted and rinsed 3 times with hot distilled water, allowing each rinse to heat for awhile on the hot plate. I also made sure the entire inside of the crucible was well rinsed down with water from a squirt bottle - otherwise, the dried silver nitrate can contaminate to gold. This was repeated with a 3/2, HNO3/distilled H2O mix. It is best to use reagent grade HNO3. However, if the technical grade contains no chlorides, it will work fine.
24-25 - I parted, dried, and annealed in the same porcelain crucible. I decanted so that the gold ended up all together in a pile on one side of the bottom of the crucible. Annealing was done on a triangle over a bunsen burner. The gold must be thoroughly dry before annealing or it will spit out of the crucible. The gold will quickly change from brown to golden color when annealing. The hot crucible was set on a firebrick, or other good insulator, to cool before weighing. If you set it on metal, the crucible will crack, with a loud pop.
I have never seen or used yellow litharge, what is the difference and why do you prefer it?
The yellow litharge is PbO and the red oxide is Pb3O4. Actually, the yellow is called litharge and the red is called red lead oxide. The percentage of lead only varies by a couple of percent. I don't really remember why I preferred the yellow, other than that's what all the books recommend and that's the way I was taught. I have used both and they both will work. I remember that there were slight differences in their usage, but I can't recall what they were.
