Floating gold is a problem that all prospectors are faced with. It can wash away all your hard-earned pay faster than you can react to it.
Let’s take a look at the composition of water. Water consists of two hydrogen atoms attached to a single oxygen atom. Although the molecules are electrically neutral, intra-molecular forces cause the electrons to spend more time near the oxygen atom than the hydrogen atoms. This gives the molecule a polar electric field. They act like magnets except the fields are electric in nature rather than magnetic. Because opposite fields attract, the oxygen atoms are attracted to the hydrogen atoms of any neighboring molecules. This causes them to line up, resisting any effort to pull them apart. This mutual attraction is called surface tension because it manifests itself on the surface of the liquid. An example of this force can be illustrated by the beading of water on a waxed car.
Gold Is Hydrophobic
Gold is hydrophobic, meaning it repels water. Because of this, even if the piece of gold is initially completely submerged, if it gets near the surface it will through off the water above it and float. The reason is that the water’s surface tension acts like the straps on a trampoline. Climb on one and they will stretch downward but hold you up. In the same way, the weight of the gold will stretch the water links creating the surface tension, but as long as the gold is lighter than the upward force from the surface tension the gold will float.
While this may be all fine and interesting, what we really need to know is what is the least amount of surfactant needed to prevent gold from floating. A surfactant is any liquid which reduces the strength of the water’s surface tension. They work by wedging themselves between water molecules, thereby weakening the forces of attraction between them. The surfactant creates billions of weak links in the surface tension.
The most common surfactant is Jet Dry. This is a dishwasher additive that prevents water droplets from spotting glasses as they dry in dishwashers. Prospectors prefer it because it not only does a good job, but also unlike dish soap, it doesn’t foam up very much. This is why we want to use as little as possible.
Bubbles and foam can support fine gold and in so doing help to wash it away. The bubbles and foam act different than surface tension but can be just as problematic. Use too much Jet Dry and you get foam. Use too little and the surface tension isn’t weakened enough and the gold will float away.
The right amount of Jet Dry to use will depend on the method of clean-up you are using. A clean-up sluice or Miller table works out to about 1/8 teaspoon per gallon of water. More Jet Dry can be used if your recovery system isn’t causing bubbling and foaming. Panners will want to use a minimal amount, probably no more than a drop or two, because the panning process will invariably agitate the water enough to cause bubbles.