PATINA – A BRONZE ODYSSEY Part 1 By Adam Clegg
A ‘SeriousWatches’ guide to forcing patina.
- Common Case metals.
- Case Finishes.
- Cleanliness / Tools.
- Patinating agents.
- Patina removal agents.
Hi everyone and welcome to our new blog, A Guide to Forcing Patina. At SeriousWatches, we are always looking for new ways to help our valued customers, so (with the ever growing popularity of Bronze and Brass watches) we think it’s time we offered you all an insight into our experience / experimentations with forcing patina. The patina we see on our bronze and brass watches is simply a layer of oxide, which darkens and protects the surface of the metal; these watch cases start to darken naturally when exposed to the air, but the process is very slow and as such we don’t always realise it is happening!
If, like us, you are a tiny bit impatient and it feels like your watch is “taking forever” to start patinating, then you’ve probably considered doing something to speed things up a little. Forcing patina can be a lot of fun and can lead to some interesting, unique and fantastic results. We’ll even give you some tried and tested methods of removing the patina, in the event that you aren’t happy with your results.
At this stage, we ought to point out that SeriousWatches cannot be held responsible for any damage caused to third party watches, should they be exposed to corrosive chemicals; however, as long as you are careful we don’t foresee any issues.
The information contained in this blog is offered as a guide only, to give you some base knowledge & pointers prior to you forcing the patina on your own watch. There are doubtless many other very experienced “chemists” who have a wealth of knowledge in this field (we’d welcome your findings, hints and tips too); please therefore don’t assume this is the “bible of patina,” see it more as a head-start to save you time and effort in learning what we already know!
Common Case Metals
The most common case metals we find are Brass, Tin Bronze (commonly known as Bronze) and Aluminium Bronze (AL-Bronze - not as common). All are copper based alloys with different elements added in varying proportions to give a range of properties; it is the copper in these alloys which oxidises and forms the dark and protective layer on the metal.
Brass – CuZn – Copper with a proportion of Zinc added. There are many grades of Brass available and as would be expected, each grade will react in a slightly different manner when we expose it to chemicals. Brass is generally a Yellow-Gold colour.
Tin Bronze – CuSn – Copper with a proportion of Tin added. Again, there are many grades available and the suffix number depicts the percentage of tin added to that particular alloy. For example CuSn8 has 8% Tin, whereas CuSn6 has 6% Tin – you get the picture! CuSn Bronze is generally a Rose-Gold colour.
Aluminium Bronze – CuAl5 – Copper with a proportion of Aluminium added. I use CuAl5 as an example, because I think this is the most common alloy used in watch cases, where 5% aluminium is added. Aluminium Bronze in its raw form is a Yellow-Gold colour.
Each of these alloys will react slightly differently to each given method of patinating, though they do mostly follow a similar path. As a general rule of thumb, the lower the content (by percent) of Zinc / Tin / Aluminium, the faster the patina will form. The photo below shows a CuSn12 watch on the left and a CuSn5 watch on the right; both are similar ages and both have developed a natural patina. See how much darker (hence the faster oxidation reaction) of the CuSn5 alloy!