Overcooking enamels on silver…

The first picture below is of a matrix of 6 rows (5 different transparent enamels from the top down, none on the bottom row) and 7 columns (6 different fluxes from the left, none on the rightmost) fired to completion at around 820C on scrap fine silver.  The second picture is of the same specimen subsequently accidentally fired for another 10 minutes at around 850C.  The five enamels were probably ruby, aqua, tangerine, reptile green and amethyst “professional jewellery enamels” from Vitrum Signum.

Thermochromic enamels

Anyone using enamels would soon find that some are thermochromic, displaying different colours at different temperatures. The pictures below show the transitions of two different enamel colours (on fine silver); the outer corners and the central square of the cross were enamelled with transparent reptile green, the remaining areas in transparent ruby.

From this one can see that when red hot, not surprisingly the whole mass glows; then, on cooling, the reptile green corner areas show as black (about 400C?), turning to amber (perhaps 200C), then yellow/grey/green (not illustrated, about 120C) before becoming a rich green when cold. The ruby cross however, although reasonably pink after a single firing, when fired several times becomes progressively more and more grey, finishing (as here) in strange fibrous opaque clay-like swirls.

Dichroic glass on ‘liquid enamels’

A rolled-out copper coin (i.e. rolled with a jewellery mill to remove the pattern and give more real estate to work on) was dipped in ‘liquid flux’, a powder which I mixed with distilled water to a suitable consistency. It was then fired, giving an unusually smooth and glossy clear coat. I then gave it a layer of ‘liquid white’, mixed from powder in the same way. Both powders had been bought from Vitrum Signum a year or so ago, awaiting a suitable time for experiment.

On firing, there was a pleasant smooth slightly matte white coating. To liven things up, I painted some little dashes of cobalt oxide in water into the surface, staining it with some dark blue patches. These remained matte through one or two more firings before starting to become glossy, presumably through vitreous material making its way through the surface oxide. Finally I laid a piece of dichroic glass on the enamel and fired that; on cooling, I found that I could ‘pop’ the top layer of glass from the dichroic, leaving an iridescent coating on the coin. Also, a significant amount of the white had been gradually dissolving into the clear flux below, leaving a bright image of the coin beneath.
The second piece, a small rectangle of copper with unwanted enamel experiments, was also treated to a small rectangle of dichroic glass fused to the surface. Or rather, two pieces, but the right-hand one slid off in the furnace.

Burn-out time

The final part of the Winter courses casting component meant that I carefully packed 20 ceramic shells in expanded polystyrene chips inside a couple of large square plastic boxes (both originally held Turkish dondorma!), and carted them off in the train to the workshop near Wimbledon.
The weather was very cold, somewhat breezy but clear when I got the furnace dome set up and the task of burning out the waxes underway. To my dismay, of the 60 or so items on top of the shells, around 9 came to some kind of grief, although later about half of these were to be repaired at least to some extent. Unfortunately one of the casualties was the wax of a 12cm pig, intended to be cast in silicon bronze. This gave an audible loud ‘pop’ inside the furnace dome, breaking into four or so large pieces.
My feeling is that the increased failure rate (about double the usual) is probably related in some way to the temperature at which the waxes had been stored, and that at which the burn-out took place. However in at least one case, the failure was due to the fact that a collection of items on a shell had insulated the outlet wax stalk from the heat; the expanding wax had nowhere to go but out through the top, bursting it off. This particular item was later given a repair.
I wondered, as on several occasions in the past, if the number of failures could be reduced with strategic additional ’sprues’ intended to allow leakage of wax during burn-out.

Carving hard plaster

I have a series of small plaster figures, each more than 5 years old, which I would like to modify and hopefully improve before recasting in bronze or other metal.  The plaster was an alpha-hemihydrate type, hence very hard.  I found it carves rather nicely, but slowly, with an HSS burr in a pendent drill.  This leaves chatter and other tool marks on the surface of the plaster, but frequently I found these marks rather appealing.  It also ‘carves’ well with a small coarse alumina-composition grinding tool in a pendent drill.  Various shapes are available, unfortunately I find that the smaller and more precise shapes are a less coarse compound and take longer to cut.

Having some of the old rubber moulds available, which I made around the same time, I tried pouring copies in a much softer (‘potters’) plaster, but the incidence of air bubbles was so high as to make the casts nearly useless.  I expect that if the unset mix, and subsequently the moulds containing the freshly-poured mix, were subject to reduced pressure with a vacuum pump, they may become just what I need.  Unfortunately I mislaid my aspirator some time ago, and this kind of cheap and effective pump now seems difficult to find.

Silicon bronze strip

By way of entertainment I soldered some scrap pieces of silicon bronze strip to some equally scrappy pieces of sterling silver.The result was then pushed through the rolling mill with some copper mesh to texture, and the surface subsequently polished a little. It looks as though it will be very suitable for suggesting to students for two-coloured rings. One caveat is that silicon bronze tarnishes rather rapidly when worn by some people, including myself. I subsequently found that some craft plastic mesh, so fine as to be almost transparent, was as good at putting an impression on the metal when rolled through the mill. My reading of Oppi Untracht’s texts suggests this is perhaps a simple version of ‘married metals’.

From chisels to gravers

Another staff development course financed by Central YMCA, I attended the first day of Working with Stone with Charlotte de Syllas as tutor, and the four days of Engraving with Wayne Parrot and Alan Craxford, both courses being held at the Sir John Cass Department of Art, Media & Design within London Metropolitan University. If time, timetable and cash had been on my side, I would ideally have also liked to do the remaining four days with Charlotte. Perhaps next time. I have already done quite a bit of lapidary work, and made a good start on fashioning a lacy agate cover for a cast silicon bronze seashell (pictures soon).
Engraving however is something I have not tried before, although I have used gravers for raising beads in pave work, and the main impetus for attending this course was to get to grips with gravers, as it were. I still regard them as one of the most difficult metal-working tools to do useful things with, and fortunately this course gave a good grounding for a beginner like myself, although it was really aimed at those who had already done a three day intro or had had some experience. But I started the course after talking to the Director, Alan Craxford, and things seemed to work out quite well. Wayne Parrot seems to be one of the few traditional fully-trained and experienced engravers around, and made an excellent tutor. Pictured below is my first crude attempt…

More work with enamels

After the end of term (apart from a Friday sculpture class) I had enough spare time to tinker with some enamels. Some ‘bronze’ 2p pieces were rolled a little to reduce the surface detail, with annealing as required. These particular 2p pieces are now becoming uncommon; more modern ones are actually made of steel with an outermost thin layer of copper alloy. I use a magnet to separate them out. A 1.5mm steel wire was polished with sand paper then coiled into a shephard’s crook sort of shape, and used between two of these rolled bronze coins to impress the design when the sandwich was again rolled through the mill. For good measure, I repeated the trick with two squares of 1.5mm annealed copper. I later used these for enamel tests. Another two bronze coins, again rolled to reduce the amount of relief design on the surface, were drifted with diamond flux before firing. Then a piece of coarse metal mesh was laid on top and opaque white enamel powder drifted on top. The mesh was carefully removed and the result fired, to give a nice regular array of bumps in white. It was then re-fired sufficiently hot to cause the diamond flux to bubble up under the white and give a beautiful sparkling gold appearance. Other areas went green / black, probably some interaction between the flux and copper oxide (i.e. scale from the first firings).

Making a silicon-bronze ring III

Although the rolled-out 0.5mm silicon bronze in part II below seemed perfectly acceptable for a ring liner, I thought I would substitute some of the fine silver I had recovered from sterling scrap. The thinking was that it was softer and would be easier to tinker with by hammering; also there would be a colour contrast, and less chance of copper staining for the wearer!
The picture on blue felt shows the colours better, but also shows that I hammered the side face of the silver liner with a ‘0′ numeral punch (the only type of small punch I possess). If nothing else, it shows that it is high time I bought or made some suitable small punches. The second picture shows the reverse, where the rivetting of the inner fine silver liner was done by hammering a conical mandrel into the ring, then burnishing the lip over for the last fraction of a millimetre. Fun, but I need to try a few more to refine my technique.

Making a silicon-bronze ring II

The insert for the ring was originally made of silicon bronze strip 13 x 0.5mm after rolling. This was cut nearly to size and rolled round in a ring former. It was then slipped inside the domed ring and the butt edge scratched with a scriber to give some idea of where to cut. In fact, I found one needed to allow about 0.8mm to get a perfect fit. It was soldered with hard solder in a similar iron wire ring to that for the dome, but smaller in diameter. An even larger gauge iron wire, springy and very tough, was used to pull this ring apart during heating to re-adjust the solder seam – very inventive, but not quite inventive enough since the ring went shooting off into the room. After re-soldering, pickling and cleaning, it was a nice fit in the domed ring.
However I felt it was unnecessarily heavy, so repeated the process of making a plain band with 13 x 0.25mm silicon bronze strip. In all cases for this project, repeated use was made of the enamel kiln set to about 800ºC for annealing. This satisfactorily softened the metal, but the darkened surface was quite resistant to removal even with hot acid, taking additional scrubbing with wire wool. Subsequently, I felt that I should remake the inner band a third time, this time wider, so that it could be riveted up the sides – the extended rim looked nice.