Some workshop notes on plique-à-jour

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:

First layer

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.

Second layer

Packed in the same way, fired in the same way. Looks like this:

After nitric, same bits of enamel and firescale as before:

After HF:

Third layer

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.

22 thoughts on “Some workshop notes on plique-à-jour

  1. I hope this doesn’t sound like a trick question, and I’m not going to cause undue angst by asking it, but…

    A fan of prehistoric metalworking asks – what’s the firescale and what causes it?

    1. Trick? angst? why? hehe

      Firescale is the oxide that forms on the surface of copper when it is heated up. It is so thick it peels off in black chunks. There is usually a lot still attached to the copper that has to be chemically removed.

      1. I’ve heard of hammerscale in connection to iron-forging – I know that copper and bronze objects get annealled after casting so I wonder if the ‘firescale’ gets removed at this stage. Hmmm, she says, pondering…

        1. I got pieces cast brom bronze and they came to me quite clean, but maybe they did pickle them. Silver comes out clean and white. I think they oxidate because of oxygen and there is none in lost wax casting (there may be in more open sand casting molds).

          Why would they need annealing after casting? Cast ingots are very soft already.

          1. Another interesting comment…

            Closed two & three piece moulds are a Middle & Late Bronze Age technology (lost wax comes in during the LBA) so maybe there’s no oxidation there. Early Bronze Age objects are cast in open moulds so oxidation may be a problem…

            The annealing tends to be done for work hardening purposes. Otherwise, the edge is too soft for, say, cutting down a tree. There’s a lot of disagreement over whether objects such as copper axes were ever actually used, as they’re probably not as useful as a flint axe.

            But they do look good…

            1. Oh, but annealing softens, not hardens non ferrous metals (and I think ferrous as well, but I know next to nothing about them). Even if you quench (lately even I’ve been told that quenching may soften even more).

              To harden non ferrous metals you need to bunch their molecules together, by hammering or twisting. If you do this in excess, the metal splits.

              Copper would get dull very fast indeed, even hardened.

              1. Ah, so this is where my lack of metallurgy shows. The annealling I presume is the reheating by putting the blade of the sword/axe whatever back into the fire.

                I suppose this is then alternating with hammering to create a harder edge. Or is the work hardening carried out while the blade is cold… I know it changes the structure of the metal…

                A little knowledge is a dangerous thing. Endless theory can never replace hands on experience…

                1. In this ferrous and non ferrous differ significantly. I don’t think it makes a great deal of difference when forging copper red hot or room temperature, as in its softness or malleability (once annealed that is, and until it needs to be annealed again). With iron, as countless blacksmiths in countless movies clearly show :D it is worked red-hot. This I don’t know if it is because the metal is most malleable when red-hot. I thought it was soft when hot and then hardened when quenched in water or oil, but I may be wrong.
                  It is very sad I know so little since my first vocation was blacksmith, but it didn’t turn out.

                  If bronze is at all like copper, which may well not be since it was greatly superior for tools and weapons, should be annealed and worked until close to desired shape, then a last round of hammering to harden.

                  What I worked with bronze, it seemed very similar to copper, but harder, which is a bitch, and also as much of a bitch to solder due to firescale.

                  1. The fact it’s so hard to work with is another really interesting thing to know – at yesterday’s conference, we were told about a recent bronze-smithing demonstration in Perthshire. Four replica bronze swords were cast out over the weekend – three of the four were a success, the last was a failed casting. Sounds like the mould was still damp when the metal was poured… The swords will have been leaded bronze – I think the guy in question’s a stickler for getting his ratios correct.

                    Blacksmith doesn’t sound like the easiest job in the world. You’d need muscles like an Amzaon for that one…

                    1. Hehe, yeah, but still in all these things ability is more important than strength, and strength does eventually develop.
                      In fact due to lack of brute force you are often obliged to become more inventive and use tools wisely.

                    2. One can actually do a surprising amount of nice blacksmithing without huge muscles- as long as one picks one’s projects, and uses a fairly light hammer. :)

                2. Also, the bronze alloy my cast-er? used may be very different to that used 2000… 3000 thousand? years ago.
                  What I got is what is used nowadays for sculptures, may have zinc and all sorts of stuff, maybe even lead. Probably optimised for obtaining nice patinas as well.

                  1. They started adding lead to the alloy in the Late Bronze Age so they could carry out more complex castings. It flows more readily in the mould, so I’m told. It also makes the alloy more brittle, as far as I’m aware.

                    The addition of zinc marks brass, I believe – in my neck of the woods, it’s a simple test that you can use to tell the difference between Cu alloy that pre-dates or post-dates the Roman invasion.

                    It’s an interesting thought that you can control the patination according to the impurities… I’ve seen all sorts of patinas in my time – Cu Cl (the powdery grey green one) is worst, because it means your object is actively corroding. I also saw a brilliant blue patina once, similar in appearance to azurite. The axehead in question had been chemically treated, and it had played havoc with the surface…

                    And don’t get me started on metal detectorists who get carried away with a brillo pad when they find something, then wonder why the amount of money they get as treasure trove is halved!!

                3. Generally copper, bronze, silver. etc. are hammered cold, because they can get brittle and collapse into grains if hammered hot.

                  Iron blends, on the other hand, need to be hammered hot, usually at a yellow to orange color for best results; they crack if hammered too cold.

              2. Yes, annealing softens metal, it doesn’t harden it. That being said, for some types of heat-hardening 9rather than work-hardening), one needs to start with a fully annealed piece for best results.

                That’s for non-ferrous. For ferrous, one anneals, then hardens, then carefully tweaks the hardness to make it suitable for what you want, usually by slow heating and keeping an eye on the colors produced.

          2. There is oxygen in lost wax casting- after one burns out the wax in the molds, the water in the investment is also gone, and everything I’ve ever seen come out of investment is dark. One then pickles them.

        2. Not usually. Usually after casting they’re about as soft/annealed as is possible. After casting they are pickled 9to remove firescale), and often tumbled or otherwise worked to make them harder again.

  2. Also, apart from the metallurgy, what I’ve learned in classes in both enameling and glass casting is that the smaller the particles, the more cloudy the results. For best clarity, one needs large particles.

    In my experience, that seems to be true; when I have a semi-opaque color, for instance, the more I wash it and get rid of the “fines”, the less opaque/more clear it is.

    1. Makes, sense, right? but in my experience, what makes the cloudiness is not the size of the particles, but the mixture of different particle sizes.

      When the large grains fuse with each other, you can see the “seams” like scars where they meet. You cannot see this with fine grains. In Bagués where I worked we ground finely all enamels for plique-à-jour. When fine, the tension of water is also much stronger, the grains stick to each other remarkably.

      You can see examples of this following this link:
      I don’t know if you have already seen all that.

      This copper is perhaps I little bit too thick. I see the flux can cope with the firescale once I close the holes pretty well, so perhaps I could do with 1.5mm, and solder two rings of 1.5 or something… I have to make more tests.
      You c

      1. Ooo! Interesting! Thanks for the info! Now I’m tempted to try doing some tests, since I really hate either discarding or using the fines for counter-enamel (if I don’t keep them for “painting”), and goodness knows teh surface tension of water is much less effective for holding larger particles (though they are still pretty small). I am now keen to see if all-fines mixes would fine clear… Thanks for the perspective!

        Personally, I’ve never had visible seams in enamels where things meet, at least not after the final firing (I have seen them sometimes when the piece is in progress).

        1. I don’t know if those fines (from sifting?) will work. I ground the enamel for this. Those fines may be too fine, we wash them away when washing colours, even when washing ground enamels, there is some fines we call “lime” that are washed away, lost. Makes up to 15% of the original weight of enamel.

          The seams are only visible in plique :) you can see a bit of it here in the transparent bits:
          Colour hides it, and opale of course, it is mostly visible only in flux, and if you really are looking. But in the ends all affects transparency.

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