I’m home rather sick, and today is about 10 degrees cooler than yesterday (26ºC), so I turned the kiln on. Thick copper… 900ºC.
I pierced and curved a very thick copper sheet (2mm) to make some tests. I made holes from completely vertical walls to different degrees of inclination. Curving the sheet didn’t significantly affect the degree of inclination. So, here goes.
Some prior observations: this project is to be a bitch because it is copper. Copper is really bad for plique-à-jour, because of the firescale. Since the holes very rarely can be closed in one fire, there will be firescale in the hole walls before these are closed.
Normally, to close holes in plique-à-jour I would use very short fires, not getting even close to fire the enamel to a gloss, because it tends to melt into a sphere, sticking to one of the walls.
But because of the firescale I won’t be able to do this on copper. I must fire to gloss so the acids I am going to have to use can’t get into the insufficiently fired enamel (which is a bit like a sponge). So I will get messy spheres of glass sticking in all directions.
This will have to be ground and thinned in hydrofluoric acid (who I only bring out for special occasions). There may have to be diamond burs, nitric acid and hydrofluoric acid in different degrees.
The idea is to close the holes with flux, and then apply the colours on top.
So, in practice:
I am using just washed Soyer 1 copper flux. Ideally I would grind it even if just a bit, but I don’t feel like it for a test. This will probably affect transparency.
Wet pack, the holes with inclination are much easier to fill. Filled holes a lot and went over the edges (will perhaps help the enamel not fall off a hole). After fire (loong) to gloss, looks like this:
I pickled in nitric. Inside the walls, where there was no enamel, some grains were attached with firescale. Hydrofluoric to the rescue. Submerged in pure (from the bottle that is, but this one has been used a few times before so isn’t full strenght) HF for a minute or so, these bits dissapeared. Walls are clean.
A couple of holes were closed already (the smaller ones). Inclination walls are best, less spheres. There is milkiness and some bubbles. Enamel might be: too dirty, too big grains, too wet when put in the kiln, just plain too thick a layer.
Packed in the same way, fired in the same way. Looks like this:
After nitric, same bits of enamel and firescale as before:
Super strong fire, until the flux deoxidised the copper. Good news is that the walls are clean after this fire. The biggest, untapered hole had the enamel crawl away. Doesn’t matter, this hole was clearly tooo big for an untapered wall.
Looks like this:
Bad news is that the enamel is still not clear after such a strong fire, though, so I will have to try other things. I will now remove it with burs/in HF and start over with ground flux.
If this doesn’t work, its probably too thick a layer, which means a very slow progression to close the holes…
Its really getting way too hot in here so I put the kiln off.