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.

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…

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 and experience 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 318ºC), 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.
Please remember you are responsible for your own safety! Lye is very caustic to skin (and in fact to 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 were cold, to avoid the risk of splattering hot caustic alkali around. Good ventilation was 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 (window) 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 some lapidary wheel somewhere or other has such a 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.

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’.