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Enlarge the crystal in 24k gold electroforming

Q. Hello everyone,

Can anyone let me know that by adding what kind of Carboxylic acid will lead to increase the crystal size formed during a 24k gold electroforming process ?

Thanks,

Henry Chow
Jewellery - Hong Kong
October 18, 2009

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Q. Hello Everybody,

To read this panel is quite helpful to all readers!

I would like to further elaborate my question: We are doing a phosphate base gold salt with Potassium Gold Cyanide as electrolyte to do electroforming process being controlled to neutral pH (at about 6.3 to 6.7) to make hollow 24k pure gold products with various shapes. We would like to know by adding what kind of Carboxylic acid or any other methods that will lead to increase the crystal size formed in the products, or to say that we would want to increase the hardness or the surface brightness of the products.

I will be deeply grateful for the help from all of you!

Thanks a lot!

Henry Chow [returning]
- Hong Kong
November 9, 2009

A. Henry,
Responders on this website may be a little leery to give you exact formulas. What if we gave you the wrong data, and your gold bath needed replacing because of it? I'm sure you see my point.
I will say however, pyridine compounds (pyridine-3 carboxylic acid) and other compounds have been proven through research and used in production. These additives are generally proprietary in relation to type, concentration, and make-up addition amounts to the bath itself. Your gold supplier should be able to pay you a visit, and get you exactly what you need for the bath. Good Luck!

Mark Baker
process engineer - Malone, New York
November 10, 2009

Hi Mark,
Thank you very much for your comment !
We will test it in a breaker. Our anode is Pt mesh.
Many Thanks...!

Henry Chow [returning]
- Hong Kong
November 11, 2009

Q. Dear Mark,

I have one question about pyridine-3 carboxylic acid to increase the crystal size formed (Effect of casting sand) in the 24K gold products. Currently our gold bath is containing the following ingredients:
1. phosphoric acid
2. KOH or NaOH
3. Gold cyanide

We really appreciate if you can help us to confirm that the pyridine-3 carboxylic acid will increase the crystal size and will have the effect of casting sand in the 24K gold products for our gold bath condition. Thank you.

Best Regards,
Philip

Philip Yung
- Hong Kong
February 3, 2010

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.

Mark Baker
process engineer - Malone, New York

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.

Pat Mentone
St Paul, Minnesota

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.

Chris Owen
- Nevada, Missouri

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.

Geoff Smith
Hampshire, England

Q. Dear Mark,

Currently we regulate the pH values based on phosphoric acid and KOH ratio. You suggest to vary plating times and current densities to determine the altered grain structure. So do you still recommend us to add pyridine compounds (pyridine-3 carboxylic acid)?

Thanks.
Philip

Philip Yung [returning]
- Hong Kong
February 6, 2010

A. Philip,
No, I wouldn't use the pyridine compound(s). As Pat Mentone points out, lower current densities would increase the grain size of the deposit. Try it in the lab to see if you can achieve this.

Mark Baker
process engineer - Malone, New York
February 10, 2010

Dear all,

Thanks so much for your advice and great support.

Best Regards,

Philip Yung [returning]
- Hong Kong
February 20, 2010

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