Many bronzes have tin in them.
The more I look at your photos, the more curious I am. You started with 6#? Do you mean 6 ounces? Why do you only have 1 liter of solution? How much acid and metal did you start with? With 20% silver and the rest of copper, etc., it would take 2.6 gallons (plus, 2.6 gallons of distilled water) of nitric to dissolve 6# of this metal. One gallon of nitric dissolves 7# of silver or 2# of copper. These figures are pretty reliable.
If you only had one liter (1/4 gallon) of acid and tried to squeeze 6# into it, the crystals are what are clogging your filter. You would also have tons of undissolved copper. If you started with even 2 gallons of nitric, you would have the same problem but it wouldn't be as bad. The crystals are copper nitrate and silver nitrate. Heat it up in a hot tap water bath and add some distilled water. Stir it. Get the crystals dissolved, allow it to settle, and carefully pour off the solution and filter it. Look at the solid material and probe it a bit. If its solid, you need more nitric. If you have gold and silver in it, you will have to go through the process I outlined in my last post.
Here's the way I would have dissolved 6# of 20Ag/80Cu.
(1) Calculate the nitric acid needed. It came out to 2.6 gallons. This is approximate - maybe, a little short. You also need 2.6 gallons of distilled water.
(2) Divide the shot or the pieces, as-is, evenly (weigh it) among 3, 5 gallon plastic buckets. Put each bucket in a plastic tray, in case of foamover.
(3) Cover the material, in each bucket, with distilled water. Add an extra inch.
(4) Start adding the nitric. Add about 300mL - 500 mL of nitric to each bucket. It should start working quickly. It will start getting hot. When it settles down, give in a good slow stir and add another shot of nitric. Repeat until a small addition gives no reaction. You may have to add a little water now and then to keep it from crystallizing. Don't add too much or you'll cool it off. Don't cool it. Let it get hot. This whole thing will take about an hour.
(5) Add the required amount of distilled water. You will end up with about 1.7 gallons of solution in each bucket.
(6) Let it cool and settle overnight. Pour, dip, or siphon the solution off of the solids. Filter the solution first and then the solids. If you have a little metal left over, you can dissolve it in a little nitric and water or save it for next time.
I see your 5 pint bottle of reagent grade nitric sitting there. It would take 4 bottles to dissolve the 6# of metal. That's about $150 - $200 worth of reagent grade nitric. If you're going to use nitric in this quantity, you've got to find a source for technical grade. In 1 gallon containers, tech. grade would cost $30 to do this job. In 55 gallon containers, it would cost $6.50. Of course, they want a $750 deposit on the 55 gal. stainless drum.
