ZIMMERMAN ART GALLERY

Patination of bronze sculptures:



Q&A with Fran Dibble

In connection with an exhibition of bronze works at ZIMMERMAN, we asked Fran Dibble to explain “patination” – the chemical process by which bronze works acquire their distinctive colourings.


With an M.Sc (Hons) in Biochemistry and more than 25 years of experience working with bronze sculptures, Dibble is uniquely qualified to explain this highly specialised process.

Z: What is the “patina” on a bronze work?

DIBBLE: Bronze is an alloy comprising mainly copper (around 98% minimum), with a few minor ingredients like manganese, lead and silicon. As the copper in the bronze oxidises, this chemical reaction produces a thin film on the surface of the bronze. This thin film is known as the “patina,” and is what forms the unique colouring of bronze sculptures.

Z: How do bronze sculptors achieve different patinas?

DIBBLE: Bronze sculptors are able to achieve different patinas – and therefore different colourings - by applying different chemicals to a sculpture’s surface. While the blend of chemicals, pigments and techniques used in each foundry is a closely guarded secret, commonly used chemicals are ferric nitrate (for browns), copper nitrates and sulphate (for greens), ammonium salts (for blues) and liver of sulphur (for blacks). As well as these, various copper oxides which have intense colour, blacks and brick reds, are sometimes adhered using the patina as a binder.

Z: How long does it take to achieve the desired patina?

DIBBLE: There are various methods of application, including spraying chemicals with a spray painting kit using a compressor with the metal heated up or by soaking or lightly spraying the bronze cold over time (sometimes several weeks) so that the patina slowly “grows” on the surface. In a foundry in Italy we visited, where they are often obsessed with using the same methods as their fore-fathers, they used the really old fashioned method of burying the bronze sculpture in sand and peeing over it to produce ammonia to colour the metal – a very odd sight indeed at the back of the foundry.

 

Patination is actually a very difficult, and sometimes irritating part of the sculpture making process. Sometimes they say, in the larger commercial foundries when they make large editions (in the region of hundreds, compared to our two or three, and when they strive to make them all the same, whereas we tend to relish the differences between editions) that you can pick the person responsible for doing the patination by their white hair and bad temper. All kinds of small things can have an effect – small trace elements in your water supply (when we have trouble, we start collecting rainwater), the humidity of the day, a newer batch of chemicals or an impurity in the metal. It is certainly nothing like applying paint!

 

Sometimes a patina can just work perfectly first go and other times it can take weeks to get it right. If we are not completely satisfied with the resulting colouration, we remove the patina by sandblasting, and start the patination process all over again.   

Z: How did the Dibble foundry develop the expertise it needed to develop the patinas it uses?

DIBBLE: Trial and error mostly, with lots of disasters in the early years.

Z: Is the patination process for small works the same as for large works?

DIBBLE: More or less, but sometimes we use lighter waxes for covering smaller works, as they don’t require the same durability against the elements as the outside pieces.

Z: For how long will a sculpture maintain its patina?

DIBBLE: Some sculptors leave their bronze works un-waxed, with the philosophy that they leave it to change with the elements, a sort of naturalistic idea of a changing artwork. Practically this means that everything placed outside will eventually go green. You often see this on older works – one example is the man on horseback outside the Beehive in Wellington (which they have recently sandblasted and repatinated). With that work you can even see the patina running down over the large base as green streaks. This is of course actually slowly using up the bronze itself - but very slowly.

 

At the Dibble foundry we wax all our sculptures, because that way the colour will stay the same as you see it when it is first in the gallery, as long as the wax layer is maintained. The wax is applied over the bronze after the patination, it is left to dry, and then is polished and buffed to a shine. It preserves and enhances the sculpture’s distinctive colourings. They have found old Greek bronzes in the sea with their patinas intact, as beeswax can be a very effective protectant.

 

For further information contact ZIMMERMAN - www.zimmerman.co.nz / This email address is being protected from spambots. You need JavaScript enabled to view it.