Copyright, David Reid
The full potential of a ceramic-shell lost-wax technique for the casting of sculptural pieces to metal can only be fully realised, and disasters avoided, when there is an understanding of the materials involved. Whereas any old wax could be used in the plaster grog method, ceramic shell requires stricter control of this basic material. The reason for this is that while a plaster grog mould sets around the wax and physically supports it up to the point when it is de-waxed, in the case of ceramic shell the opposite occurs. It is the wax itself, in this case, which must support the delicate pre-ceramic coats as the mould is being built up. Eventually, of course, the mould gains its own mechanical strength by being converted to a true ceramic in the firing stages.
As an example of how misleading these names can be, we just have to look at what effect the ambient temperature can have on their properties. A wax that is perfectly good for use in ceramic shell casting in the summer will become harder at room temperature as we head towards winter, finally becoming so brittle as to be useless for our purposes. How do we get over this? My advice is to brew your own. This may seem a bit daunting, especially if you don't have a degree in wax chemistry, but don't be discouraged. It's really very simple. A wide variety of waxes or oil-based substances can be melted together to give something that will serve our purposes. You'll soon learn what's useful, and what's not. If we arrange the waxes that are available to us in order from hard to soft, no matter what they may be called or what non-sculptural use they may have, we can build a useful range of waxes with the properties we require. The list can include any non-volatile, oil-soluble substance, for example commercial investment waxes, paraffins, candles, hard microcrystallines, rosins, soft microcrystallines, petroleum, oils. Many of these may already be complex mixtures. All we care about is their properties and their feel as they arrive in our hand. Above all we want to know if they are hard, soft or somewhere in between. This is not quite the whole story, as toughness and plasticity are also important, but we'll come to that later.
A good place to start the brew is with the microcrystalline range, produced in vast quantities by the oil industry. These are readily available through specialised suppliers e.g. Tiranti's, and are not too expensive. Lets say it is mid-summer (may it always be so) and the wax as purchased is a little soft (easy to push around, but tends to distort on standing.) We melt down the microcrystalline (see the section on melting wax below) and add a little of a harder wax, say one third by weight of paraffin wax, melted, for convenience, in a separate pot. A toughener and plasticizer such as pine rosin at a level of, say 10%, could also be melted and added at this point. Stir all this together, cool a bit of it, and try the mix for feel. Still too soft? Try adding a little more of the paraffin, or maybe a little less of a harder wax. Finally, fully test the mix by pouring some of it, when it's near its congealing point, into a sprue mould to make a bar of wax about 1/2" in diameter. Let this cool to room temperature (by immersion in water if necessary) checking it's no longer warm by touching to the lips. If you can detect any warmth, let it cool a little longer or you'll be led to a false result in the following test: bend the sample quickly. If it snaps, you're not there yet and are going to have problems later in the casting process, so add more softener to the blend and test again. If, however, it feels strong, and shows no sign of cracking, you are well on the way.
For a wax to be useful in ceramic shell casting it should be as hard and tough as possible, but without any sign of being brittle at the current ambient temperature. These qualities are necessary because later the wax must be strong enough for build up of a ceramic shell without deforming. I suspect this is where many founders who had trouble converting from the old Italian plaster-grog method went wrong in their first attempts at ceramic shell. They forget that whereas any old wax will be braced by the setting of the plaster-grog block, in the ceramic shell process, it is the wax that supports the mould until it goes to the kiln for de-wax. Don't be worried if your blend has the above attributes but feels too hard to model easily. It can be brought to a suitable state by slight warming, and will, of course, return to its strongest state when it cools: a very useful property.
Now, this wax we've made, fine in summer, is going to be next to useless when winter comes around, because with the drop in ambient temperature it will have moved towards a more brittle state. This is easy enough to remedy. Just remelt your batch and add more soft wax, again melted in a separate pot. Test it again. With a bit of fiddling, your wax will be re-tempered to suit the cooler conditions. These simple empirical methods free us from total reliance on what is available for sale. We can adjust any wax to suit our individual requirements.
Most waxes as they come out of the box are quite light in colour; even translucent. This can cause real problems when modelling and detailing, because we can't see the surface properly. We tend to look through it slightly, and the result is a headache. To overcome this, we can add some sort of dye to the molten mix to turn the wax opaque. The colour is not important, although I haven't met many people who can deal with, say, lurid orange when they're working on a serious piece of sculpture. Oil soluble dyes can be obtained from wax manufacturers and candle makers shops in any colour, but to get started, a more easily available source is drawing crayon. These will dissolve easily, if they're not the pigment filled type, and give the required opacity. Black is probably the best, as it is very intense, often giving a green when dissolved. Try a few available brands. Oil paints can also be used, and I've even torn apart marking pens (a messy business) and thrown them into a mix when desperate. The most usual colours I aim for are brown or deep green, since these give me a link to the finished bronze; but really, its a matter of personal preference. (even orange !)
I've found a good source of heat for a small set-up to be a small camping stove run from a gas canister. This will serve for melting, heating wax sheet and warming any tools. Later, as you learn a few tricks you may want to go to a gas ring and bunsen burner, an old stove, an electric soldering iron, whatever. At the moment we're here to get you started. Now, cooking pans and kettles (being designed to hold and pour liquids) make fine wax pots but once again stay out of the kitchen. Second hand stores will have them quite cheaply, and they may also have other handy items like scrapers, stainless steel knives and thermostatic electric frying pans that might come in useful.
We have our slab of wax, pots, stove etc., but we have a problem. No way will a slab fit in a pot. So we hit it with a hammer to break it and find, if the weather is a bit warm, that all we succeed in doing is leaving hammer marks in it. So out with the cheese cutter, knives, axes and saws and with luck we have bits small enough to fit. But there is an easier way. Chill the block in the fridge/freezer for a while, then a tap or two with the hammer will have pieces all over the place! Wax can be frustrating to use at first, but with a few tricks you'll soon have it under control. Heat the wax in the pot, together maybe, with harder or softer waxes, and a pigment. If there is a crackling noise, there is probably some water in there too, but it should should soon boil off. One thing not to do while this is going on is wander away and do something else. There's nothing more dangerous than forgotten wax that gets overheated. If you spill it on yourself it will cause a bad burn and very hot wax is likely to burst into flame. If by chance you do have a pot catch fire, or even just get very hot and start smoking, dont panic. Just turn off the gas, put a lid on the pot to exclude the air it needs to burn, and leave it alone until it cools. Don't try to pick up the pot to shift it outside as one tiny spill on your skin could make you drop the lot. Now this dangerous situation is only going to arise if the wax gets far too hot, so keep an eye on it and don't let it get out of control. Kept below the temperature of boiling water, wax is quite safe and as you'll find, a very useful material.
A handy device for keeping wax molten at a usable temperature is the electric wok or frying pan. These come with a thermostatic control, so there is less chance of the wax over- heating. Try that second-hand store, because you're not going to want to stir-fry in that pan again. When the wax is completely melted or, preferably, just before, reduce the heat, stirring once in a while. If you don't get the wax hotter than you need it, you won't have to waste time waiting for it to get back to a workable temperature.
You can get a good idea of when wax is at the right temperature for a particular purpose by watching it carefully as it cools. The first sign that liquid wax is getting close to being useful is that, on tipping the pot slightly,a thin but visible film stays on the side. If you dont see this, or if the wax is showing any sign of smoking, its far too hot. As the wax cools further, the film will become visibly thicker with each tip. These observations give a visual guide to the useful temperature range of molten wax. Other signs will also show up as the wax gets cooler: for example, an oily appearance and a hazy film on the surface indicates that it is not much above its melting point, while a ring congealing against the pot at the surface shows it is cooler still. I'ts worth learning to read any of these signs so that you can note them and correlate them to the way the wax works for you in a certain situation.
Now you can think about pouring it into sheets, a good starting point for modelling. Wax poured against almost anything dry will stick really well; so well that it is often used as a glue in cardboard box manufacture. It doesn't mix with water though, and any substance that is absorbent can be dampened and used as a mould. So damp concrete, wood, cardboard, plaster, cement sheet and clay all work as long as the wax isn't too hot i.e. below the boiling point of water. To make things easier, you may wish to put a border around the edge of your sheet, thus making a shallow tray, although if your wax is cool enough (just molten) this shouldn't be necessary. If you have a water bath big enough, a damp board or wooden tray will float level, making it much easier to pour sheets of even thickness.
So, with the molten wax showing signs that it is getting near to setting, we can pour it onto our damp surface, held level of course, to form sheets ranging from 2mm to about 5 mm - the actual thickness will depend on the temperature. As they set, immerse the sheets in the water and float them off the surface they were poured on. In this way you can build up a stock of strong, plastic, coloured wax sheet.
The next important trick to learn is how to weld wax. Good welding gives mechanical strength, an important factor when we come to handle larger sprued assemblies at the ceramic dipping stage. A weld joins two pieces by melting them together to form one. To make a strong joint is not as easy as it seems. Begin by holding a metal tool over the flame until it gets very hot. You can also use a modified soldering iron with a blade fitted into a slot sawed into the bit. Take the two pieces to be joined, checking that they are cool by touching them against your cheek or lips. Place them in position and run the hot knife tip quickly into and out of the joint, being sure that both pieces melt to some degree and withdraw the knife immediately. The pieces to be joined have to be held completely still for a few seconds while the weld sets. If your hand shakes while the weld is going from liquid to solid, the joint will be weakened. One reason you want the wax to be cool before you begin is that cool wax will absorb heat from the joint more quickly and save you having to it hold still for too long. If a weld fails at first attempt, there's no point carrying on until the surfaces are cool again. On difficult joints, holding the pieces in place with clay or some other support can be of great advantage. The weld doesn't have to be perfect. If the joint is first tacked together, it can be made stronger at a later time. Once a weld has taken, put it aside to cool and set completely before working on the piece further. Because of necessary delays while pieces cool and gain strength you will find working on two or more pieces at a time, and swapping from one to the other, to be a great advantage. Once the joint is stable and strong it can be completed by using the hot knife to run wax into any gaps left. Extra wax can be brought into the area by dipping the work blade into wax or running wax down the warm blade to where it is needed. To reinforce the joint you may wish to apply a bandage of warmed, and therefore softened, tough wax around it, welding it in place. But don't forget, welding means to melt both pieces being joined into one.
When first met, wax can be a frustrating modelling medium. This is mainly due to its inherent stiffness; you can't push it round as fluidly as you can clay. However it does have advantages. It retains much more detail, does not dry out and, as it is stronger, allows more delicate working. And, of course, it can be completely burnt out of a mould. Believe me, once you get over the initial hassles, you can be hooked on the medium. It only takes a couple of hours. Take a piece of wax and hold it over the flame. You'll note that not much happens except that the outside layer gets very soft and melts a bit. This certainly hasn't made the wax plastic or mouldable. What you are seeing demonstrated here is that wax is not very conductive, i.e. that it doesn't allow heat to penetrate rapidly. So how do we soften it down to make it plastic throughout? Two easy methods are available. One is to leave it in a warm place for quite a long time (hot water cupboard, sunshine, near a light bulb, in a bucket of warm water). Alternately use wax sheets, like those described previously, which are thin enough to be brought to a workable plasticity very quickly, by waving them over a flame for a few minutes on both sides. Heat can penetrate the surface of the sheet and with a bit of manipulation the whole mass can be softened. As long as you keep working it, it will stay pliable. On standing, it returns to being strong and tough, as you designed.
So, using this warm and therefore plastic piece of wax, you might evolve, say, the torso of a small figure. Arms, legs and head can be modelled separately and welded into place. The whole may then be textured by a spreading motion with a hot knife and gradually the integrated form evolves. Or you may decide to build directly from sheet. A word of warning, however: when you join pieces together they must be firmly welded, i.e. the piece as a whole must be mechanically strong, to allow the application of the ceramic coats later in the process. Details can be added later, but it is essential that the piece has mechanical strength of its own.