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Name: Max
Status: student
Grade: 9-12
Location: England
Country: England
Date: April 2007


Question:
I am a student studying alchemy. I would like to confirm my thoughts on the subject by asking: Is it possible to make pure gold (crystal structure face centred cubic FCC)? I do not think it is as I would have thought gold is a pure element and therefore it cannot be copied by alloying etc.?



Replies:
Indeed you can make or grow gold crystals without too much trouble assuming the proper supplies and training. The process of making the crystal will often purify the gold. You can make a crystal from it by either melting/annealing or by chemical deposition from solution. The first method requires a supply of reasonable quality gold and a torch (among other things). melting the end of the wire will cause the gold to bead up and form a liquid ball. If the liquid ball is cooled slowly (and in a uniform direction) then if has a reasonable chance to crystallize in a single lattice. Often in practice you will get a twin or triplet, but there are a few tricks you can use to improve your chances.

The surface may not show much or any direct faceting (even through a microscope), but an x-ray diffractometer can quickly verify its condition. During the heating and melting process many of the impurities will be burned off by the torch and many of the others will be forced to the surface during annealing. Repeated application of the heating and cooling can be used to improve the purity. Heating and holding the entire crystal at just below the bulk melting point of gold will also improve the quality.

You can also create gold crystals just as you would salt or sugar crystals, though you use particular acids instead of water. There are two ways to do this, either by creating a super-saturated solution directly by temperature differences or by changing the concentration (evaporation). The gold then comes out of solution (slowly) and crystallizes. Depending upon the conditions, you can many (thousands or more) tiny crystals, or just a fewer but larger ones.

In our lab, we are very interested in the properties of gold and gold crystals (and other metals). On several occasions we have actually made our own crystals by the first method. Typically we can get them up to around 1cm on side in raw form.

Most gold that you find around you is alloyed to some extent. Among other things this is done to harden the gold into a useful state. It will also be polycrystalline. Pure gold crystals are extremely soft and very easy to destroy. They really are not much stronger than a warm stick of butter(maybe that's not exactly true, but they are darn soft). A fall of just a few centimeters is all that is needed to turn a gold single crystal into a mashed up gold poly-crystal.

Michael S. Pierce
Materials Science Division
Argonne National Laboratory



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