Preserving heat coloring of copper when clear coating

A. Hi Paula. It seems to me that there would be museum techniques that would preserve your copper art as it is, probably including either sealing them in a nitrogen environment, or including volatile corrosion inhibitors (VCI) within the framed areas if a real hermetic seal weren't practical. Good luck.

Regards,

A. Hi. I'd try applying sodium benzotriazole [on eBay or Amazon] as this is a preservative rather than a clear coat, or I'd try incorporating a little VCI hidden behind the glass in the frame. Obviously, experiment with scrap rather than precious works.

Regards,

Hi John. PVD machines run about a million dollars and are out of the range of most of the hobbyists posting on this thread :-(

But yes, they are used to apply titanium nitride, zirconium nitride, and similar "lifetime finish" to brass faucets and door hardware. Thanks.

Regards,

A. I have an answer for holding color in torched copper if it has not been answered here already. It's a new product called "ColorLoc" made by Sculpt Nouveau and listed under sealants. It's meant for this purpose. You put a layer of ColorLoc on first and finish it with a layer of Clear Guard.

Hope this helps

Mark

A. I hope that the following information will be useful. It will be most relevant to the subject of copper and patinas changing colour over time. It's a bit long (sorry) but I think, and hope, it contains some helpful bits, or at least interesting. Thank you.

The British Museum in London, UK uses wax [affil links] for much of its metal pieces. For example, when ancient metal (iron or steel) armour plating has been cleaned and restored, Renaissance wax maintains the metal surface in its pristine, restored condition and, if applied correctly, does not alter the degree of shine much.

Also in the UK, local authorities are some of the biggest users of Renaissance Wax. They apply it to bronze statuary, displayed on street pedestals and affixed to exterior building walls, to maintain the patina and protect it from the elements all year round. As you might know, the UK weather is amongst the most changeable on Earth - temperatures from 30+ to -10 or lower, centigrade, and dry, humid, frequent showers and rain, snow, hailstorms. It can change on a daily or weekly basis depending on the jet stream and other factors.

I don't know if there is much copper statuary exhibited outdoors in the UK, so I can't give any information about that.

However, about 10 years ago I did a lot of conservation work on three Nepalese copper Buddhist statues for a UK Buddhist charity. Parts were heavily gold plated using the ancient copper-mercury gilding process. Other parts were beautifully patinated. There was a lot of contamination due to surface etching with nitric acid which had not been properly removed from surface pores. Remnant mercury salts, nitric acid and various compounds were leaching out, threatening to eat the statues from the outside. Inside, "bronze rot" had taken hold and was escalating rapidly in the British humid atmosphere, threatening to eat the statues from the inside. Also, remnant acids trapped in pores inside the states were already eating their way through the statue, causing tiny holes to appear on external surfaces. After removing as much of the corrosion and residual chemicals as possible I sealed the inside of the statue with a "vapour barrier" composed of an acrylic formulation dissolved in acetone; I then loaded the nasty solution with glass "micro balloons" to help the barrier solution work its way into crevices, surface pores and various welding faults. Several layers produced a completely impermeable layer against the atmosphere inside the statue. A similarly treated copper base plate was then resin bonded to each statue, the resins being loaded with mixtures of copper and bronze filings for colour matching. So that fixed the inside. Now for the outside.

First, I filled all holes with my acrylic solution or two-part resins loaded with metal powders and/or earth pigments for colour matching - tricky work. Then I painted the entire surface with an acrylic solution - except the face and other parts that had been mercury-gold plated or painted with a pure 24 carat gold powder "kind of gouache" and precious stone pigments. Then, the entire statues' surfaces, except the precious stone and gold pigment painted areas, were treated with 3 applications of Renaissance wax.

The unpredictable risk was whether there were any chemical contaminants inside the copper casting underneath the painted areas, which I couldn't seal externally. If anything broke throughout these areas, the beautifully delicate painting would have to be removed, then repainted using pure gold gouache, which isn't available, so has to be custom made by experts, and no-one in Europe (and probable the USA) makes gold powder fine enough - the finished gold gouache has the same reflectivity as human skin.

If any atmospheric contamination penetrated the vapour barriers, or if I had trapped any significant amount of humid air in the copper surface defects, patinas would change colour, copper would oxidise - except where I had used metal powder loaded resin fillers - thus, if the copper changed colour, my patch repairs would become increasingly obvious.

After a couple of years or so, I returned to apply more Renaissance Wax, "to be sure to be sure". Everything seemed to be good.

I have checked with my client, now and again over the years, the last time, after 10 years, in July 2015. There has been no noticable change. I know 10 years is nothing in the world of renovation, conservation and art, but the statues have been displayed in a bright, well light room, handled on a regular basis by non-experts for regular dusting and cleaning, and have been transported often around the country to be displayed in all sorts of well lit environments. Transportation has usually been simply wrapped in silk and put in a box.

So ... I know the details of vapour barriers inside the statues are of little relevance to jewellers and surface-artists, but I thought I'd describe in detail the problems I faced, which you will probably agree were extreme to say the least. I think the processes most relevant to this thread are:

1. If an effective vapour barrier is not applied, chemical change and discolouration will happen.

2. If any residual chemistry is not washed out - and that means washing chemicals out of the tiny surface pores created by acids and other corrosive chemicals - then chemical process will continue below an effective vapour barrier until the chemistry has "run out". In this case colour change is likely - but will continue if corrosion causes a chain reaction - that is, where a chemical process causes by-products that are also corrosive. For example "bronze rot" is a chain reaction wherefrom the atmosphere must be completely removed unless your piece can be expertly treated by lengthy chemical deep soaking. I couldn't deep soak the statues because I had to retain delicate gouache paint decoration. In your case you want to retain the patinas, which deep chemical soaking usually removes.

3. An acrylic vapour barrier is effective, theoretically - it requires very painstaking application. Usually, however, it is not sufficient to apply just one layer. In my case, several layers would completely destroy to external appearance. In any case my reason for using acrylic solutions was to try to completely real most surface defect. So I opted for final treatments with Renaissance wax.
In conservation a general rule is, "every process must be reversible". As I mentioned, many of the British Museum's metal artifacts, and bronze street sculptures in the UK, are treated only with Renaissance wax.

All my formulations were considered in consultation with the metals conservation department of the British Museum, and the wonderful experts at Conservation Resources Ltd, in Abingdon-on-Thames, Oxfordshire, UK, from whom I purchased all the necessary chemistry except resin fillers and metal powders which I purchased from from a sculpture materials supplier, Alec Tiranti in Thatcham, Berkshire, UK. Please note that The British Museum is not chartered to provide free consultancy for businesses or individuals who sell their products. A Registered Charity number for a non-profit organisation is required.

There will be good materials suppliers in your own countries. I have found them to be happy to examine artifacts and discuss solutions; just find one that is not just a retail outlet, but which has in-house expert conservation chemists.

A. Hi Yael. I don't know who George is, since there are no postings with that name on this page, but I don't think it is wise to try to make a patinated copper vessel food safe with a clear coat.

I've tried to explain why every clear coat will always change those patina colors: it's because the colors result from light bouncing off the outer surface of the flame patina, and also bouncing off the inner surface (or copper surface below the flame patina) and the two reflected lights interfering with each other. When you add a clear coat layer, now there is another surface the light is bouncing off of, so the result will always be different than before the clear coat.

Regards,

A. Hi Debbie. Paints and lacquers are generally not water proof, but slowly breathe moisture in and out. In fact, if they are too waterproof, that can be one cause of blistering.

I'm not sure that I fully understand your description, and don't really have experience with this, but a cooler door is cool and is constantly pulling moisture out of the air as condensation. This probably is the cause of those sparkles appearing on the cooler door and not elsewhere. I am not familiar with commercial coolers as in your question, but home refrigerators have tiny heating coils in the door which you can turn on to stop this condensation.

Regards,

A. I flame torch copper and have had luck with ColorLoc by Sculpt Nouveau; it locks in those blues and purples. It's expensive, not the product, but it costs the same amount to ship as the price of the product -- but worth it.

A. Hi I have just finished torching some copper sheets to use as decorative risers on a staircase. Got those great blue sand gold tones that I was looking for. Then wiped over with CorrosionX and retained almost 100% of the colouring. A lot easier than half time powder coat clear mentioned above.

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  Hi Robert. I suspect that your process is not only easier and cheaper, but works better initially. The test will be how long it lasts. Try to remember to get back to us in a year. Thanks.

Regards,

Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
September 2018