My two cents
I incinerate my powders in a corning casserole dish, then let them cool some before washing the powders (these powders will stay in this casserole dish throughout almost the whole process).
Incinerating the powders will help to oxidize base metals, it can also remove organic material and oils, tin is a big problem in refining gold, it does not dissolve well and can lock gold up in solution, the incineration helps also to oxidize the tin, so that it will go into a hot solution of HCl and can be washed away from the gold.
The boil in HCl after incineration helps to dissolve base metal that have been oxidized in the roasting incineration process (when these powders were brought to red hot glow in an oxidizing environment such as air or better yet oxygen added to the red hot powders, stirring to expose these powders also helps), the boil in HCl will also dissolve tin as well as copper oxides, lowering heat after a good boil and letting powders settle but keeping the solution hot helps to keep these metal salts in solution, while letting the gold settle from solution, just a small amount of water is added (I use a spray bottle which can help sink any floaters, I do not add too much water as this can make the tin chloride less soluble (it becomes milky in solution, and is harder to filter), I decant the solution while still very warm using a suction bulb then a pipette, this solution is run through a filter, or set aside in a settling jar (some value may have traveled along for the ride), I judge by the color of this solution whether I need another HCl wash, if solution is dark brown (lots of copper and other metals ), another HCl wash, If very light green or blue then I will go to water washes, covering the valuable powders in water and bring to a boil, lower heat to settle powders, but hot enough to keep the lead chloride soluble in solution, decant this washes above and repeat until water wash clears, lead chloride is insoluble in cold water, but is somewhat more soluble in boiling hot water, silver chloride will stay in the powders if silver was involved, a wash in house hold ammonia (ammonium hydroxide), will dissolve the silver chloride, just warm the solution we do not want to boil off the ammonia solution, let powders settle and decant ammonia solution to a jar, add HCl till solution becomes slightly acidic (silver chloride will precipitate from the ammonium solution when HCl is added),
Note:as soon as ammonia solution was removed from gold powders cover the gold powders with HCl, this step is to make sure you do not dry the gold powders in a basic state with ammonium compounds (for your safety) it will also again help wash the gold powders, this wash I dilute more when I decant the wash.
CAUTION do not let powders dry that have been treated with ammonia, always acidify them with acid HCl in this case, and rinse before drying, temperature or shock sensitive compounds could form from silver in ammonia solution if powder was dried without acidifying.
Few more water rinses till water decanted is clear,
Then we can go directly to the aqua regia process, no harm.
Or you may prefer to use a nitric acid wash to remove silver instad of useing the ammonia wash I discussed above, before you dissolve the gold in aqua regia:
Note: if you were going to go to a nitric solution to remove silver, (unlike the process of ammonia I used above), or to get silver and palladium from the powders, then we cannot go directly from a HCl solution or wash, as we have formed chloride salts in these powders, and by going directly to nitric, the acid in nitric would dissolve these again into solution with the nitric acid and dissolve gold or even some platinum powders with heating, so to avoid this we need to neutralize the powders with a solution of sodium hydroxide. (my reasoning for neutralizing and rinsing these salts is to remove chlorides as much as possible gold chloride, or gold in salt is volatile at high temperatures), powders neutralized (can test with pH paper or meter), and salt solution rinsed in hot water, powders dried and incinerated to remove traces of chloride salts, cooled then we could do our nitric acid washes.
You can go to aqua regia after nitric acid wash no harm.
The few drops of sulfuric acid that has been suggested, is discussed in Hokes book, it is to remove traces of lead from the aqua regia, the few ml of sulfuric also is useful when evaporating the aqua regia solution, the sulfuric (which will not boil from solution) helps to keep salts from forming if you evaporate further than you need to (also helping to keep you from scorching your gold if evaporated to far by mistake), the sulfuric in solution will do no harm, (in fact if you use ferrous sulfate to precipitate gold from aqua regia your making some sulfuric in solution with HCl in that process).
Another very fine trick is the one we learned from Harold, using an added gold button in our aqua regia solution to remove excess nitric from solution, with this trick you do not have to go through the laborious task of three evaporation's, just a concentrating of solution for the free nitric to be consumed by the added gold button (which how much of this gold button that actually dissolves is retrieved, and is a good indicator of how much over use of nitric acid we had, speaking of this when using aqua regia to dissolve our gold it is best to add slight excess of HCl and heat and add nitric in small amounts, giving the gold time to react with the heat, and add nitric in small additions to avoid adding too much nitric (the goal being to use up all of the nitric in solution to dissolve the gold and leaving no free nitric acid in solution to have to evaporate (this and Harold's gold button trick, can help to make things much more simple in the life of a refiner)
