Enlarge the crystal in 24k gold electroforming

A. Philip,
I haven't had any experience with the bath you are using. If the bath contains phosphoric acid, NaOH or KOH, how are you buffering the bath to regulate pH values? I think you are better off doing lab scale testing concentrating on varying plating times and current densities to determine the altered grain structure you are looking for.

A. Dear Philip

Rather than messing up your bath with chemicals, you can control the grain size by the type of rectification.

The hardness of the deposit is inversely proportional to the square root of the grain size.

So for harder deposits you want a fine grain, for soft deposits you want a large grain.

High current density produces fine grains, low current density produces large grains.

Pulse plating at low duty cycles (10%) gives the finest grains,DC plating at low current density gives the largest grains.

A. Although I am probably out of date, the proprietary pure neutral phosphate (usually phosphate/citrate) systems I have used contained a grain refiner. If I remember right, thallium sulphate was commonly used for this. Even so, I have never seen deposits from these baths that were very bright, at least compared with the common acid citrate baths that contain small amounts of Ni, Co, or In. It seems to me that, with the phosphate system, the brighter you make the deposit, the greater the stress you will have. And, of course, the higher the stress, the less chance electroforming will be successful.

Have you tried one of the proprietary sulfite gold baths? Some of the formulations are pure, very bright, and very ductile, and they are excellent candidates for electroforming.

A. Henry

Jewelry is never made of 24 kt gold, it is far too soft and wears rapidly. Pat is right, you should be looking for smaller grain size, not larger.

However, I believe that you will waste time and money by trying to re-invent processes that the industry uses every day.

Speak to your gold supplier. They can not only provide extensively researched (i.e. stable) processes but can match the colour of the traditional jewelry golds. If their process ever goes wrong they will help you fix it - well worth the price of a phone call.