Enamels on copper plate on silver

Various pieces of PMC had areas that were electrolytically copper plated, then covered with areas of reptile green and ruby red transparent enamels. On firing the results were mediocre, however with hindsight I realise that I chose two unpromising colours - in particular reptile green goes brown on copper, rather than the beautiful green it goes on silver flux on copper. I then went on to embedding small copper shapes in PMC and applying enamel, though in most cases coping with the oxide was the biggest problem.

Detection of mercury vapour

After some research in my chemistry books, I settled on cuprous iodide / silver iodide paste mixed with precipitated sulphur as a possible reagent. The reagent was made by double decomposition of silver nitrate and copper sulphate solutions added to a solution of potassium iodide. The resulting yellow paste was painted onto filter paper and allowed to dry. On exposing to mercury vapour in a glass vial, the exposed area turned a satisfying but not very visible light amber colour. I doubt that it would detect the vapour from the mercury spill which occurred some years ago in the kiln room...

Over-cooking enamels on fine 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. 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, all from the "professional jewellery enamel" range 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 corner squares and the central square 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 areas turn from red to black (perhaps about 400C?), next turning to amber (around 250C?), then yellow/grey/green (not illustrated, around 150C) before becoming a rich green when cold. The ruby 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.

Detection of lead in dust samples

I relied on literature in school chemistry books which proposed acetic acid. Once dissolved, the presence of lead is easily detected by adding a solution of a dilute soluble sulphide or polysulphide, giving a dark-brown / black precipitate of lead sulphide. However in my experiments with small fragments of cleaned lead strip, I found it did not dissolve in acetic acid even in high concentrations over a period of 8 hours. Heavily corroded lead also did not dissolve. Further reading suggested that dissolved atmospheric oxygen is required. Adding a small amount of hydrogen peroxide rendered all lead and at least some corrosion compounds readily and rapidly soluble in acetic acid solution. The problem now is that any precipitate of lead sulphide tended to get oxidised to white lead sulphate by surplus peroxide, compromising and complicating the original test scheme. However I did manage to show the presence of lead in 0.1% lead acetate solution used as a control , although it was more problematic in 0.01% solution.

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.

The eyes have it...

The piece on the left is plain fine silver (from a rolled-out fine silver casting grain) with a faint leaf-vein pattern hammered in, and some enamel on to test colours. It was very boring, so I fused a pair of fine-silver eyes with purple enamel to the surface.
The right-hand piece is again fine silver, PMC this time, moulded from a real leaf (starberry). This gave the veins in reverse which was more attractive than the original. After enamelling, I thought it also benefitted from a pair of eyes, perhaps I've just seen too many cast leaves...

Glass enamels

I found a few pots of "glass enamels" which I had bought some two or three years earlier from a company called Potterycrafts, when I was thinking of doing some more glass fusing. Checking in the catalogue, I found that the firing temperature was just under 600C. Five samples were made, and two illustrated below;

I added a semi-abstract semi-pastoral image in primary red, blue and yellow to a discarded piece of copper which had a grey enamel surface, and fired it at 800C, out of curiosity. To my surprise, the colours didn't burn out, but gave the image shown. A second enamel piece, this time with a rather nice abstract leopard-skin pattern, had some spots added in the same primary colours, and fired at 600C. The result is the piece in orange / brown with darker spots.

The glass enamels were extremely easy to paint after adding enough distilled water to make a thin cream; so I am considering getting some "painting enamels" if possible, which are presumably equally painterly in effect and intended to be compatible with metal rather than glass. There have been no signs of distress on the enamels above.

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.

Removing enamel with molten alkali

The details for removing enamel with molten lye are frequently kept hidden on the grounds that the technique is dangerous. My own assessment is that it is comparably dangerous to leaving work on Oxford Street in London on winter evenings, given suitable safety training for both activities. Other people's assessment may differ.
I cooked the object with the enamelled areas covered with about an equal volume of anhydrous sodium hydroxide ('caustic soda' or 'lye'), at about 400ºC (which is above the melting point of about 360ºC), in a tiny iron container (an old pot lid) in a small enamel kiln. I imagine a tiny copper pot would work perfectly well. The molten lye dissolved the enamel within minutes, and was then allowed to cool when it set rather rapidly into a grey-green mass. When sufficiently cold I irrigated it with a large amount (a bucketful) of cold then boiling water to dissolve the caustic soda together with reaction products. Usually I found that the enamelled object would be stuck to the pot with solid lye underneath, hence the boiling water to help dissolve it quickly.
See the footnote to this website for the disclaimer concerning safety! Lye is very caustic to skin (and in fact all human tissue) even at low concentrations and temperatures, so I wore goggles and rubber gloves. Solid cold lye can misbehave with water since it is an exothermic reaction, and I found that molten lye misbehaves even worse (explosively possibly) with even very small quantities of lye - hence the reason for waiting until all reaction products are cold, to avoid the risk of splattering hot caustic alkali around. Good ventilation is required - a small amount of injurious alkaline spray is produced which one wouldn't want to breathe in, and more if the alkali is not cold when added to water.
But, the pay off - the vast majority or all of the enamel simply dissolved away, and in those cases where some remained, it was removed by drying carefully then repeating the operation.
An alternative technique uses an aqueous paste of sodium chloride (common table salt) and potassium sodium tartrate (Rochelle Salt), applied to the enamel areas, then the whole heated to 750ºC. It is then plunged into cold water ('ice cold' is recommended on various sites). I found this was occasionally very successful, but sometimes only as good as plunging red hot enamel into cold water, and that it often never removed all the enamel even after repeated application. The use of such high temperatures and high temperature gradients on quenching is also likely to be problematic in some cases, causing warping for example. The chemistry involved seems to be unknown - the enamel is not dissolved, but seems to break away from the substrate more easily.
I also briefly experimented with molten potassium hydroxide for removing enamel with satisfactory results. The melting point is similar to that for the sodium salt; and the boiling points, at above 1300ºC for both, are sufficiently high to guarantee a stable temperature region of molten alkali. The potassium salt is of course potentially more reactive, and poses greater safety risks. It didn't seem to improve the removal of enamel, but was more effective than the sodium salt at removing ceramic shell investment from an old bronze specimen I had had lying around for a few years. As expected, it also severely degrades glass.

Meccano meets Anglepoise

I have only had time for a few small tinkerings over the past couple of weeks; these included trying to cast pewter in a way that embeds copper alloys for contrast (the idea came from Oppi Untracht's book and mostly failed, I need a means to secure the copper to the walls of the mold first); carving a design into a gesso panel using standard gravers (went very easily when the gesso was dampened); carving a discarded slab of leather-hard clay into a primitive Gothic arch; and making a very "Heath Robinson"-contraption to help when hand or machine grinding of cabs and other stones requiring a precision flat face. Well, if not precision, then at least not rounded too much. In use, the stone is attached to the base of the left-hand vertical of the jig with setter's wax or similar, the right-hand vertical is ideally screwed to a work bench or immobilised with a weight on top, and the parallelogram arrangement ensures that the lateral grinding action of one's hand is regularised to prevent rounding of the bottom face of the stone. And it works, and even better when the stone is actually on a lapidary wheel. I imagine professional lapidary wheels have a rather better jig built-in. The white blob riding on the sandpaper is actually a thermoplastic, which is a very bad idea since it is as tough as nylon and greatly impedes sanding / grinding; setter's wax worked much better, both by hand and on a flat lap. I have actually thought of what might well be a better system for hand grinding on wet-and-dry, but not got around to making it yet.

A brief holiday

From 4th to 16th September I had a holiday in southern Turkey; mostly staying in Antalya, with a day at Phaselis, and 4 days at Kaş, all three on the coast. Very enjoyable, it's been a long time since I had more than a long weekend anywhere, although I never acclimatized very well to the daytime temperatures of 39C (or the night-time ones of about 28C). My thanks go to Kemal Özkurt of The Owl Bookshop, and to Harun Enez and Hussein Enez of Boncuk Bazaar (both in the Kaleiçi in Antalya) for looking after us so well.
Anyway, I found to my surprise that my brain performed noticeably better than it usually does (as measured by scores on Sudoku and Bobby Carrot [a Java game on my mobile], and remembering events for my diary!) which highlit the extent to which I have been flogging the poor thing.
Perhaps now I'll have the energy to actually make something, instead of watching others make things.

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 had intended to use these for enamel tests - see the picture in the right-hand bar.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). See picture in the right-hand bar.

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.

Making a silicon-bronze ring I

I cut two rectangular strips of silicon bronze, 37 x 1 x 12mm, then wished I had super-glued them together for the subsequent filing square of the ends and the long ends. The original stock strip was 1.5mm, annealed and pickled, rinsed and washed, then rolled to 1mm. It was a great help to hold the emerging end of the strip with parallel pliers as it came from the rollers, my hand holding the pliers elevated with a suitable board and movement facilitated with a sheet of plastic underneath. Even better probably would be a suitable emergence table at the exact right height. I used dividers to mark the strip, at 37mm and 38mm for sawing in between; and placed a strip of balsa wood on the bench peg to help steady the strip for sawing. This was done twice to give two rectangular pieces 37 x 12 x 1mm, they were superglued together and left until dry in the vice. Then they were filed as necessary for squareness, before placing in the enamel kiln at dull red heat to break the bond and anneal.
For doming, I bent a piece with pliers until it fitted into the largest dome, domed it, and repeated until I had reached the 26mm dome, the smallest hollow that this size strip would still fit inside. The strip, on bending slightly, now sat in the 25mm dome with 2mm protruding, therefore there was a line inset 2mm from the edge caused by the last doming operation. The resultant ring was oval at 25 x 25.5mm. Sanding the short edges by holding them flat on sandpaper (painful on the fingers, especially if using a steel block to try and ensure registration with the other dimension) got it to a nice circle at 25mm diameter. My feeling is that using the steel block is a waste of time, and doesn’t help any kind of squaring operation. However it is possible that the rounding of the corners of the short edges using the block was entirely due to swapping faces pressed against the block, without having taken steps to absolutely guarantee that the two faces are exactly parallel. So I suggest use face A on block, edge on sandpaper; then edge on block, face A on sandpaper; then edge on block, face B on sandpaper; but never face B on block and edge on sandpaper – this would accumulate an error! Mark near face B with ink to reduce the chance of a mistake.
For soldering, I hit on a happy idea – a circle of strong steel wire stuck out in space, the two halves of the ring aligned and dropped into this slightly smaller circle to merely sit there. Then solder from the base – this worked perfectly.

Cleaning sterling grains

The sterling grains I had made (by re-melting and pouring into water about 500g of silver from a previous failed casting venture), gradually looked very dirty on the surface. They were steeped in sodium carbonate solution sat on aluminium foil for some days, which made them noticeably worse. Subsequent steeping in sulphuric acid made no change; nor permanganate, peroxide or bisulphite! I melted one of the larger lumps with some borax to get an interesting set of colours; white as in clean sterling on the part in contact with the crucible bottom, black on most of the top as in oxidised sterling, and a fluorescent yellow / green (which had been very golden in colour when red hot) on part of the top, possibly caused by the borax flux. The lump of silver was dumped in sulphuric acid for a couple of hours. Incidentally, these grains had been satisfactorily used to cast student work in early spring.
Subsequently I placed all the remaining tarnished grains in a ceramic shell open mould and set it in the enamel kiln at around 800°C and left it to reach dull red heat. On removal, as hoped, the surface of all the grains had changed from a kind of creamy mushroom grey to dark grey. They were cooled and dumped into dilute sulphuric acid, which satisfactorily converted the surface into the kind of clean white I associate with sterling silver.