When we drop gold, especially from a dirty solution, it can pick up other metals that may be in solution. When we wash it, it can mean water, especially hot or boiling water. This will remove certain contaminants that are water soluble, but leave some that aren't. So after a water wash we use an acid wash, usually hydrochloric, to remove any metals that came down with the gold that water can't deal with. Nitric acid as well as sulfuric acid can be used, and may work better at times depending on the other metals in solution. Here is how I wash most of my dropped powders.
Allow the powders to settle well in the beaker. (can take hours if not days)
Decant off the AR with out disturbing the powders to much.
Boil in water, 2 or 3 times. (Decant the water between heating and start with fresh water.)
Add acid and and allow to sit while cool for 10 minutes.
Decant this off and add new acid, this time add heat to just under boiling.
Decant and add new acid and heat to just below boiling.
Decant and add water and boil. Do this again two or three times.
So long as the water becomes even slightly cloudy or the acid changes colors repeat alternating between the water and acid wash until both remain clear. I do a water wash last to remove all traces of acid, then gently heat in the beaker until no moisture condenses on the sides of the beaker and steam is coming from the powders. I don't use a cover as we want to completely dry the gold at this point. I don't boil as this can cause gold to fly out of the beaker in small steam pockets bursting and send it flying. Just enough heat to see the steam and see moisture condense will get the job done. When it is dry, the color can give you a good idea of purity. The paler the color usually the better the gold. Use a clean dish and melt your gold.