Standard methods of sweeps burnout are ineffective and produce a product similar in appearance to lava rock, and which is more, rather than less, difficult to refine. The following method requires about the same amount of work as ineffective methods but reduces to sweeps to a material that can be melted to obtain a bar of metal.
Polishing sweeps are composed of grease (the binder in polishing compound), sandy grit (the active ingredient in polishing compound), lint (from the buffs) and very tiny balls of karat gold (from the gold that was polished). Sometimes it will also include paper and buffs.
The first step is to remove all organic material. Take the dust and put it in a series of casserole dishes. The dust must not be more than 1" high and it must not be packed into place. This is very important. If sufficient air does not reach the sweeps, they will not burn completely. If there are buffs and large amounts of paper, these must be put aside for a separate burning.
Place the casserole dishes in your burnout oven. You can stack them, but make sure that there is sufficient space for air to freely circulate through them. If using an electric burnout oven, the door must be left slightly ajar so that sufficient air enters the oven.
Burnout at 1350º F until it has stopped smoking and an additional hour has passed. When it has cooled and you examine the sweeps, you should observe no blackness and no lumps. Typically, it is a uniform, light gray, fine sand. If examined a loupe, you will find tiny balls of gold scattered in the sand.
If you attempt to melt at this point, the sand will turn to glass and its sheer volume will overwhelm the metal and microencapsulate the gold (you’ll get a lump of black glass will little beads of gold throughout it). To avoid this, you must first remove most of the sand. This is done with lye (also called sodium hydroxide or caustic soda).
Lye is very corrosive, so take the normal precautions that you would take with any corrosive material. Wear rubber gloves, eye protection, and clothing protection.
Make a saturated lye solution by adding just enough water to the lye to cause it all to dissolve. Lye gets hot when you add water, so use cold water.
For every cup of polishing sweeps, add 10 cups of lye/water solution. Heat the solution to about 200º F for about 1-2 hours in a stainless pot. Do not use aluminum or any other metal; the lye will rapidly corrode aluminum.
Allow to cool to about room temperature and then carefully pour off the lye/water, being careful not to pour off the gold particles in the bottom. Rinse by filling the pot with fresh water, allowing the gold to settle and then pouring off the water. There will still be a lot of sand but it will not longer be overwhelming. The gold will be visible. Dry by placing the pot back on the hot plate at low temperature.
If melting by torch, first wrap it tissue paper and soak in alcohol. Use a partially covered crucible such as a Burno crucible or a casting crucible. These steps will help prevent the gold dust from being blow about by the torch.
The resulting gold bar may now be refined either in-house or it may be drilled for assay and then sent out to be refined by professional refiners.
Typical return of gold is 1 ounce per pound of polishing dust. This usually represents an increase in return of from 2-10 times the return over both non-processed and incorrectly burned sweeps. In other words, if you don’t know what’s in your sweeps, you’re losing your shirt every time you send them in.