Noxx,
I don't like the filter paper routine. It's hard to do accurately and it can be dependent on the type and quality of filter paper used. Instead, I would use a 30 ml Coors porcelain crucible to dissolve the silver and then just pour the solutions off - To me,this is a lot easier than using filter paper. You can use a small beaker for this, but it's easier to see the small black specks of gold in the white Coors crucible and thus not pour them off along with the acid. Also, you can't anneal the gold in a beaker.
Also, instead of the standard 3/1 inquartation, I would use 5 or 6 to 1. The gold is more apt to break up but, by using a weak nitric first, this can mostly be avoided. Also, using more silver will help insure that most all of the silver and base metals will be leached out of the gold with the nitric.
When pouring the acids and subsequent rinses off of the gold, hold the crucible with tongs and pour the acid down a glass rod which is touching the rim of the crucible. That way, the acid will flow more evenly and there will be less of a chance to lose any gold. There's a picture of this in the fire assaying chapter of my book (which you have) on page 48. Before pouring out the acids or rinses, swirl the crucible gently and tilt the crucible a bit to collect all the gold together in one spot at the bottom edge of the crucible. When pouring out the acids or rinses, go slowly so the gold remains at the same spot without moving. If the pile of gold separates, stop pouring and swirl again to re-gather it.
Although it's not as accurate as cupeling (the results will normally be slightly high), because you don't get rid of the base metals before treating with nitric, I have seen people that routinely inquarted the karat gold with silver with a torch. When I didn't have a furnace or lead to cupel with, I tried it a few times with fairly good success. That is the method I will outline here. The most accurate way, of course, is to wrap the sample and the silver in assay quality lead foil and cupel it normally to eliminate the base metals. Whether you cupel or use a torch, use the same procedure as outlined below.
A small electric skillet is ideal for heating the acids and rinses in the crucible. A Corning Ware dish on a hot plate also works well.
Keeping all this in mind, here's what I would do. When it calls for water, use distilled water
(1) In a new cupel (preferably a magnesia one, since they are much more durable than bone ash), alloy the karat gold ( weigh out .25 to .5 grams and record the weight) with a torch so you will have a 5 or 6/1 ratio of silver to gold. Make sure everything is collected into the bead - no stray BBs. Remove the bead from the cupel with the tip of a small pair of electrician's pliers (they are the best type of pliers for this use - just grab the bead and twist to break it loose) and squeeze the bead somewhat with the pliers to break up any cupel material clinging to the bottom of the bead. While still holding the bead in the pliers, use a small toothbrush-size wire brush to gently remove any cupel material. Flatten the bead as much as possible on a clean anvil with a clean ball-peen hammer.
(2) Heat about 20 ml of 1:7, nitric:water in the crucible until you can see some steam. Add the flattened bead. Heat until there is no more reaction (no bubbles coming off the gold). Don't boil. With the tip of a stiff, small diameter, stainless wire, press on the center of the gold. If it is hard, there is still silver in it. If the silver (plus the base metals) is gone, it will usually break up into 2 pieces. If there is still silver in it, add 2 or 3 ml of concentrated nitric and continue heating until there is no more reaction.
(3) Carefully pour off the acid without pouring off any gold. Add about 10 ml of 3:2, nitric:water and let it react for about 10 minutes. Get the acid quite hot but don't boil.
(4) Pour off the acid and repeat.
(5) Pour off the acid and add about 10 ml of water from a squirt bottle while rinsing down the entire inside of the crucible. This will rinse down any silver nitrate solution clinging to the inside of the crucible. Heat (don't boil) until steaming. Pour off and repeat this rinsing 2 more times.
(6) After the final rinse has been poured off, the gold should all be together in a pile on one side of the bottom of the crucible. Carefully, without disturbing the gold, place the crucible on a warm hot plate (skillet) and heat until dry. If you get it too hot, the gold will splatter. Dry it slowly. After you think it it dry ( the color usually gets a little lighter), raise the heat and really make sure it is bone dry.
(7) The next step is called annealing, Since it calls for getting the gold red-hot with a flame, you can't do it in a beaker without it breaking. You can do it in the Coors crucible over a bunsen burner or by gradually heating it with the slowest flame possible from a cheap propane sweating torch. If you use the torch, heat it slowly or the crucible will crack. Whether using a bunsen burner or a torch, just heat the bottom corner of the crucible below the gold until the gold is red-hot. At this point, the gold will lighten in color considerably.
(8) Place the crucible on an insulating firebrick and allow it to cool. If you put it on metal or a cold surface, the crucible will crack.
(9) Weigh the gold and calculate the purity.
