Under-fired 2020 Flux Enamel on Copper

In my last post, I mentioned that there is a very large difference in appearance between under-fired and fully-fired clear enamel on copper. In this example, I used Thompson 2020 flux over copper.

When the under-fired piece came out of the kiln, the copper oxides were black and then cooled to the red colour captured in the photo. If I fully fire the copper, the oxides will be absorbed into the glass and the metal will appear bright once more.

Left: Under-fired 2020 flux over copper                                                                                                             Right: Fully-fired 2020 flux over copper

Penny Colour Tests

The other day when I was at the bank, I noticed an elderly gentleman in the line before me had just deposited a giant box of Canadian pennies (which are no longer in circulation). Canadian pennies made from 1942 to 1996 have a very high percentage of copper (98%), which makes them excellent for testing enamel colours. When my turn at the bank counter arrived, I sheepishly asked if I could get $2 worth of the non-circulating pennies that had just been returned. The teller was wonderful, and said ‘sure’ without a blink. I felt badly for asking her to count out 200 coins, but the allure of easy enamel samples outweighed my embarrassment.

Transparent enamel colour tests on copper

The designs on the surface of the copper pennies make testing my current transparent enamel palette easier, because it allows me to judge which colours require a layer of clear flux enamel over the copper, and which colours can be applied directly to textured metal. This way, I can predict which colours will be effective with textural enamel techniques such as basse taille.

Transparent enamel colour tests on copper

A few years ago now, I purchased a sample set of Thompson enamel that included all of the medium expansion enamels for use on copper, gold, silver, low carbon steel and metal clay (SAMP-C, for those of you who might be interested). At the time, I made a decision to use only transparent colours, because I loved the play of light on the surface of the metal. I made fine silver test strips which I keep in my sketchbook, but I didn’t take the time to do copper tests. Tomorrow, I’ll finish doing the last 12 transparent colours in my regular transparent enamel palette, and then I will begin testing the opaque colours from the sample set.

Transparent enamel colour tests on copper

On another note – I find it very interesting how differently fine silver and copper behave when they are enamelled. I ended up firing my copper penny samples up to 1550 ⁰F, whereas for silver I would never go above 1510 ⁰F. When I’m working with reds, oranges and pinks on silver, I try to keep most of my firings around 1495 ⁰F so I don’t burn out the colour. Copper seems to require a higher firing, because it produces oxides at high temperatures that need to be absorbed by the glass enamel before the colour will reveal its true nature. It is particularly obvious when a clear flux enamel on copper is under-fired, because the undissolved oxides produce a very red colour beneath the glass. I’ll try to get a photo illustrating the dissolving copper oxides tomorrow.