Lately I have been getting engrossed with kiln casting in glass. Aye, this is unusual for a goldsmith. Alas, our kind is known for getting obsessive about detail, due to doing work on a very small scale and with precious material. Lost wax casting with rubber moulds is appealing to the nerd in me – and may enthrall others!

Sacrum Quiver, body adornment in blown and cast glass, Patricia Niemann 2011/12 (image by Fergus Mather)

I personally will have to blame a masterclass attended at North Lands Creative Glass in the summer. Michael Rogers from the US, glass artist of international renown, teacher and poet, led a truly inspirational class. Also, I have been drawn more and more to anthropology and the study of the human body, and this particular way of working is the only way to reproduce my chosen models faithfully. And, finally, my work is … err … somewhat increasing in scale.

What follows below is an illustrated description of the process, lengthy and still in no way complete. How did this come about? Well – Twitter is to blame! Or more precisely my Twitter friend Justin Bellinger in Denmark. During the masterclass I ‘mini-blogged’ a little about the process. Talking to him it became clear that the process was too complicated to explain in a few tweets 140 characters in length. So I promised him a proper explanation – and only recently followed up on it: I have an important solo exhibition coming up shortly at Broadfield House Glass Museum in the Midlands and needed to make more pieces: A perfect opportunity to recap the process! The sacrum is a bone located at the bottom of the human spine, forming part of the pelvis. My piece also contains the coccyx below.

Lost Wax casting: A Human Sacrum

Making a rubber mould

1. Preparing the sacrum (medical specimen)
The sacrum bone has to be looked at carefully: Are there undercuts and hollows, which make moulding difficult? There are all of those! These have to be blocked off and smoothed down with oil-based clay (plastiline).

2. Framing the sacrum

Now, the sacrum is embedded halfway up in a layer of more plastiline (green in the image). A big cone-like shape is made from plastiline and attached to the sacrum – this will later be the reservoir for the glass to be molten into the final mould. A tight ‘frame’ is built around everything with sheets of glass cut to size. This frame has to be watertight, because liquid rubber is to be poured in later: All edges are to be sealed with more plasticine. Deep registering indentations are made with the help of a dowel and the whole interior of the frame is sprayed with a separator (that’s why it looks so shiny here).

3. Pouring the first layer of rubber

Liquid silicone is carefully measured, weighed and mixed with catalyst and booster and slowly poured into the frame from the highest point. While it takes time to set (30 min +), the surface of the rubber must be painted with an old brush to prevent it pooling in the lowest spot – and try to even out the complete rubber thickness all around the model. From previously set rubber chamfered bits are cut and attached into the setting rubber (later registration marks for plaster).

4. Pouring the first layer of plaster
Now plaster ismeasured out, a water/plaster mix made and poured to about 3cm thickness over the rubber and left to set. The plaster layer will later give stability to the completed rubber mould.

5. Turning the mould and treating the opposite side

The frame is now to be turned onto the plaster side and the plastiline on top and around the sacrum is removed. The sacrum is left in place. A new half of the cone shape is modelled from plastiline and attached. Again, the surface is carefully sprayed with separator and another layer of rubber mixed and brushed on (with chamfered bits). Another layer of plaster is poured and let set. Finally the whole mould can be disassembled: First, take the glass walls away, and then take off plaster casings. Now prise the two rubber halves apart carefully and off the sacrum bone. Finally remove the sacrum bone. The rubber will have picked up the finest details of the bone surface!

Making waxes

1. Pour wax

Assemble the new hollow mould, taking care to fit everything together by using the registration marks. With strong rubber bands the mould can be kept tightly pressed together. Level the mould on the table. Melt moulding wax and pour/ladle into the cavity. It is best to leave the wax to set undisturbed for 4 hours at room temperature. Then disassemble the mould again. Special care must be taken in prising off the rubber, especially when you are aware of special ‘undercut’-issues: the wax is soft and fragile and can break easily. The sacrum is such a delicate piece, that something always breaks or needs fixing. This can be done with the help of metal dental tools and the flame of a wee spirit burner or candle.

2. Finish wax

The finished wax on the far left has wax sprues attached. These ensure that the glass can later flow into the extreme fine ends of the mould, which could otherwise be closed off by trapped air in the mould. Visible in the image are also the original sacrum bone in the middle and both halves of the two part rubber/plaster mould in the background.

Making a plaster-silica mould

The wax in the image above sits stuck on a piece of float (or window) glass. This arrangement is to be transferred onto a small pottery turntable. Enough plaster and silica flour are weighed out in equal proportions, thoroughly mixed by hand and set aside. A face mask must be worn, because silica dust has the nasty habit of gradually blocking up the lungs forever, when inhaled (‘silicosis’). A small bowl of shredded glass fibre is also prepared. Lay a throwaway bristle brush at the ready and off you go: Pour a small amount of cold water into a small bowl and with dry hands start drizzling in the mould mix until a mound forms in the middle. Let this soak through and mix or ‘squeeze’ to a consistency of cream. Wash the mixing hand and start painting the mould mix quickly and evenly onto the wax, taking great care to reach and cover any nook and cranny! Start at the bottom of the cone ‘reservoir’ and work your way up. The fist coat is crucial and hard to apply. The mixture tries to ‘pearl off’, but it will ‘stick down’ eventually. Once the mix is too firm to paint, pour the excess into a lined bin and wash the wee bowl in a dip basin. Plaster or plaster mix must never be poured down the drain, because it will gradually block pipes. Continue to mix fresh material and paint on even layers until you have reached an even thickness of about 2cm all around the wax. Now add the fibre to a new mix and spread it on evenly. The fibre will strengthen the mould when firing. Finish the mould with two more coats, checking the thickness on the bottom intermittently by lifting the glass plate and peeking under the wax. Smooth down the last coat as evenly as possible using your (wet) hands. Let the mould set for a little, but no more than a couple of hours. Steam out the wax as soon as possible, i.e. before the mould has dried. At North Lands the mould is set on a metal table with a hole, through which the hose of a wallpaper steamer comes up. The molten wax drops into a basin below, filled with a little water. Cover the mould with a plastic bag, tie loosely and steam away, for ca. 1 hour. The wax is molten out completely, when the top of the mould feels hot. The mould is now at its most fragile. Let it cool slightly before moving and place to dry in a drying kiln or in a warm place, always with its opening down. The moulds will feel significantly lighter after drying.

Firing the mould

Now you can see what the cone reservoir is for! The cold casting glass (Bullseye billets in this case) is weighed, cut to size and very carefully loaded into the moulds. The moulds are soft, the glass is hard and sharp, but no speck of mould wall must fall down into the mould! If it does, it will be imbedded in the cast forever. The filled moulds are levelled and stabilised with ceramic props and kiln furniture and sand is put down on the kiln shelf to catch any potential disaster overflow.
Now the kiln can be slowly fired up, at a rate of 50°C/hr to 840°C. At this temperature the glass is liquid enough to fill up the whole sacrum cavity over the space of 4 hours. Liquid glass behaves like honey. Because glass is an insulating material with tricky expansion and shrinkage rates, the cooling must be very slow and controlled. Even when the temperature meters suggest safety, the finished cast pieces should sit undisturbed for a day to make absolutely sure they are evenly cold all the way through. These pieces will be in the kiln for 72 hours altogether, 48 hours of which is firing and controlled cooling.

Finishing

1. De-vesting

This image was taken after the firing, once the kiln has cooled completely. You can see that all the glass has molten down into the mould.

In order to de-vest the piece, i.e. break the mould away and clean the glass, a work area needs to be prepared. A few layers of newspaper are spread out and wooden tools and spatulas prepared. A metal brush (not too stiff and hard – your glass piece is delicate), old toothbrushes and inexpensive small bristle brushes will be helpful. Latex gloves are useful: The mould material tends to dry out your skin. Make sure you – again – wear a tight-fitting facemask because of the silica dust, and ideally work under extraction or outside: The fine dust will get everywhere! Carry the now soft and brittle piece to the prepared work area and carefully start easing the mould material away. It will come away in very convenient layers. Note the paper thin edges on the far right in both pictures above: This is very thin, sharp and dangerous glass. If you are careless or get distracted, it can be (by experience) a serious potential hand-slicing hazard. The thin remaining layer of mould material can now be tackled with the metal brush, then the toothbrush and finally the small bristle brush. At this stage a fine metal dental tool may be helpful to remove the last trapped mould material. Generally, it is best to clean the piece of all mould material before bringing it in contact with water; even if it seems tedious. On the right you see one of the almost completely de-vested sacra. The sprues have done their job perfectly!

2. Cold working

This is done by machines or by hand using abrasive material and water.  Water not only cools the piece, but also binds silica dust particles generated in the process. Safety glasses are essential for any kind of cold working: Due to the process and the fast spinning machines tiny glass chips are created, which can bounce far. These machines make a lot of noise: Hearing protection is advised. The first machine used in this instance will be an even-edged diamond-sintered grinding wheel on a lathe: Notches are carefully ground to mark intended break-off points in the sprues. The next machine will be the sliding table saw: It is a big and very loud machine, seemingly scary, but in reality one of the most ‘harmless’ and useful machines in a ‘Cold Shop’. Here is a picture of the two sacra after the reservoir cone and the sprues have been sawn off. The next cold working stages will happen on a lathe with stone or diamond wheels in different profiles and ever finer grit sizes.

On the far left you can clearly see one of the sacrum’s holes. The other holes are not so obvious and not all completely free of glass. Earlier, I may not have been careful enough with the painting-on of the first mould mix layer! Alas, it turns out that the glass blocking those holes is very thin and can easily be removed with small diamond burrs using a pendant (flexible shaft) drill machine.

Glass casting is not always this complicated, but in the case of this piece lengthy and necessary. Most of the stages are somewhat meditative, and if care is taken at every stage – hugely rewarding! Studio glass is an entirely addictive art form. You have been warned.

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