Professional Jeweler Archive: Turning Sweeps into Gold

June 2002

Professional Bench/Refining


Turning Sweeps into Gold

By better preparing your polishing sweeps, you can increase your return of gold two to 10 times, says this refining expert


Standard methods of sweeps burnout are ineffective and produce a product similar in appearance to lava rock, which is more – rather than less – difficult to refine. The following method requires about the same amount of work as ineffective methods but reduces the sweeps to a material that can be melted to obtain a bar or button 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 the sweeps 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-in. high and must not be packed into place. This is very important because if sufficient air doesn’t reach the sweeps, they won’t 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 there’s sufficient space for air to circulate freely through them. If you use an electric burnout oven, leave the door slightly ajar so sufficient air enters the oven.

Burnout at 1,350&Mac251;F until the sweeps have stopped smoking and an additional hour has passed. After cooling, examine the sweeps. You should observe no blackness and no lumps. Typically, the residue is a uniform, light gray, fine sand. When examined under a loupe, you’ll see tiny balls of gold scattered in the sand.

If you try to melt at this point, the sand will turn to glass and its sheer volume will overwhelm and microencapsulate the gold (you’ll get a lump of black glass with little beads of gold in it). Instead, 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 you would with any corrosive material (wear rubber gloves, eye protection, etc.). Make a saturated lye solution by adding just enough water to dissolve all the lye. Use cold water because lye gets hot when mixed with water. For every cup of polishing sweeps, add 10 cups of lye/water solution. Heat the solution to about 200&Mac251; F for one to two hours in a stainless pot (don’t 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 no longer be overwhelming. The gold will be visible. Dry by placing the pot back on the hot plate at a low temperature.

If melting by torch, first wrap the residue in tissue paper and soak in alcohol. Use a partially covered crucible such as a Bruno crucible or a casting crucible. These steps will help prevent the torch from blowing gold dust about the area.

The resulting gold bar or button may now be refined in-house or should be drilled for assay and then sent to be refined by professional refiners. Drillings should be taken at three places in the bar or button. Convey to your refining company that you have excellent, independent knowledge, through assaying, of the true percentage of pure gold in your metal. It doesn’t matter whether you send the drillings for assay – it’s just the message you want to send to your refiner.

Typical return of gold is 1 ounce per pound of polishing dust. This usually represents an increase of two to 10 times the return of 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 to be refined.

By Peter Shor

Peter Shor is the son of Nathan Shor and is a member of I. Shor Co. Shor International Corp., Mount Vernon, NY; (914) 667-1100.

Copyright © 2002 by Bond Communications