LOST-CLAY MOULDING and
LOW-TEMPERATURE KILN-CASTING :
TWO REVOLUTIONS FOR PÂTE-DE-VERRE

by Frédéric Morin and Salomé

In 1995, when beginning alone with pâte-de-verre in the aim to produce glass proofs of the famous sculptor from Luxembourg Lucien Wercollier (Philippe, 1995, 239-240), I have been lucky enough to fit my first kiln dedicated to pâte-de-verre with a two kilowatts heating element only. That made the process ceiling 740° centigrade for 24 hours (for I was not present), the worst temperature, for the Corning optical glass B2359 employed, or so usually considered about crystallisation risks.
This dwell at 740°C temperature is only 26°C or 4 percent above the Littleton Point. The common knowledge ensures that this range of temperature has to be passed across as quick as possible unless we make crystallisation appear. This is the reason why kilns are usually fitted with 32 kilowatts heating elements (instead of my 2kW) and why the kiln is opened after the process to cool the glass inside the plaster moulds.
When my first cooking came cold, I felt amazed that the enclosed sculptures were not crystallised but perfectly clear, suitable for optical polishing.

Salomé joined the glass-studio in 1998 as a sculptor. She developed there a new process of modelling clay around a core of polystyrene, in a lost-clay process similar to the lost-wax technique employed both for Bronze and Crystal.

The different attempts we made afterwards ensure us that this crystallisation temperature can be regarded as a point in the curve much more than a range. This felt as evident when we managed, a couple of years afterwards, to avoid crystallisation in rising the upper temperature as slow as possible, we mean 4°C per hour as a maximum for the last 150° before the highest temperature of process : then we could stay as long as we wished, much more than 24 hours if necessary, without any trouble. Similar experiences have been conducted with other kinds of glass —we mean with the barium crystal provided by Philips (The Netherlands) or sodo-lime industrial glass by T.G.I. (Germany) that we are employing now — with similar results.

This unexpected original success opened onto a wide range of possibilities, regarded by Dr Giuseppe Cappa as able to revolutionize the art of statuary if taken into consideration the innovations raised by Salomé (Cappa, 2001, 30-31).

We developed a new conception of the process as a whole : first a sculpture is made out of clay pasted onto a polystyrene basic figure. This core of polystyrene (Fig. 1) allows to approach the form with a low cost substance, both soft and rigid.


Fig. 1 : A core of polystyrene allows to approach the form with a low cost substance, both soft and rigid ; photo Frédéric Morin

Clay is pasted on (Fig. 2, 3), kept as wet as possible and modelled with wooden tools as well as with table knives with different kind of teeth.


Fig. 2 : The clay is pasted onto this first core of polystyrene, it may stay as soft and liquid as required ; photo Frédéric Morin
Fig. 3 : From the detailed pictures, Salomé is finishing the clay ; photo Frédéric Morin

This clay coating is supposed to remain around one centimetre thick (Fig. 5), in a way similar to the flexible silicone or elastomere skin around the polyurethane hard-core of a socks-mould. Nevertheless, the sculptor is free to add much more clay than expected as well as to excavate the polystyrene if the sculpture requires to do so.


Fig. 4 : The finished sculpture before moulding ; photo Frédéric Morin

Here, when rising more than four hundreds sculptures including human bodies and animals, we had this trial with the portrait, that we expect full of living light. We took eight pictures of my father Claude Morin (fig. 1), who created the first independent glass-studio in Europe in the year 1970 and had in Eastern 1973 the visit of Harvey K. Littleton and Sybren Valkema together with his students of the Rietveld Academy in Amsterdam.
At that time, Claude Morin (whose hobby was previously to sculpture) made Sybren’s portrait out of clay, made a print of plaster of it, and blown glass in this print (Kermer, 1993, 43, fig. 16). When attempting for portrait, it was natural in a way to celebrate our predecessors (fig. 4, 12).


Fig. 5 : The sculpture is still in the plaster, the figure shows that the clay is just a skin around the polystyrene, which is first destroyed ; photo Frédéric Morin
Fig. 6 : Pressed-air is useful to remove the finest part of clay that would remain from the removed sculpture ; photo Frédéric Morin

When this sculpture made out of clay and polystyrene is finished, the refractory plaster will be cast around it. The original sculpture will be destroyed inside the plaster : first the polystyrene is scratched into grains with a fork, by hand or using pressed air, until the clay becomes visible. When the clay is to be seen, that means that the plaster is not much distant (Fig. 6).

The clay is bent into the space left by the polystyrene. Pressed air is useful to clean of the plaster from any remain. When cleaned and dry (or so regarded), the mould is set into the kiln, the sculpture is upside down. A common plant pot (of clay) is fitted above the opening of the mould (fig. 7), and filled up with optical glass lenses (from Corning) or rods (from T.G.I.) on which colours are sprayed (fig. 8).


Fig. 7 : Plaster moulds and their upper plant-pot receiving uncoloured glass rods and colours are fitted into one of our electrical kilns. Note the bricks that will offer an extra-volume to fed retractions ; photo Frédéric Morin
Fig. 8 : Coloured glass reduced in powder is sprayed onto the rods to colour the further casting ; photo Frédéric Morin

This technique allows large dimensions (our highest sculpture was 1,50m) and dynamic positions standing out of the plumb : the core of polystyrene forbid the wet clay to collapse during the work.

Some tricks have been developed to allow thin parts or inaccessible ones when cleaning the print of plaster, such as the horns of a bull, or the bill or wings of an eagle. Then the polystyrene is left in place, but previously polished with the wheel and covered with adhesive tape to give a regular print, or pasted with lard instead of clay. The final result will appear similar, although these materials will burn into the mould. If the cooking is long enough, the glass will progress into the accurate shapes thanks to the vent-holes that make the air get out, and the burning will not affect the glass with the same reducing effects as the lead-crystal would be affected with.

Although we reach two hundreds cooking and four hundreds sculptures, we continue to note carefully the way we fill the pots and the exact curve followed. A computer will lead the temperature up to 740°C at the speed of 4°C per hour for the last 150° : then the glass will collapse from the pot through the increased hole and fill the print during the 10-20 hours dwell. The annealing temperature (545°C with this Corning optical glass) is reached two days later at a similar speed of 4°C per hour and the dwell will be one day long.

The present curve we employ now with the T.G.I. glass is :

T°C length speed length
wished ramp dwell

160° 6h 24°/h 12h
600° 30h 15°/h 0h
755° 48h 3°/h 6h15
562° 48h 4°/h 24h
462° 48h 2°/h 0h
362° 36h 3°/h 0h
200° 30h 5°/h 10h
10° 30h 6°/h END

As a whole, our process is fifteen days long : it takes five days to reach 750°C, then sixteen hours of dwell, followed by ten days cooling. When cold, the empty pots are removed, the moulds lifted out of the kiln with a crane, the plaster moulds destroyed with a wood-saw and hammer (Fig. 9). The glass-sculpture will be then cleaned with water and the feeding cone will be cut with a diamond saw, this consisting the basement of the final sculpture. Nothing but a careful cleaning, using different bronze or plastic brushes, is required to deliver the sculptures such as you can see them.


Fig. 9 : Dismantling the plaster mould is easy with wooden saw, hammer and brushes that are enough to clear the final skin of the sculpture ; photo Frédéric Morin
Fig. 10 : After the process during which the glass contained by the plant-pot has flown into the mould, retractions of the glass between 750°C and 560°C are something such as 20% : here, the process level (750°C) was much higher than the final lever (562°C), about one litre was lost ; photo Frédéric Morin

The low temperature process discovered by Frédéric Morin (750°C with optical or industrial lead-free glass) produces amazing results in the high quality of prints of the finest details of the original skin of the clay sculpture, such as the restitution of the cracks of the clay under a wooden tool that reproduces the detailed effect of a feather for instance (Fig. 10). The medium viscosity of the glass forbids it to wet the plaster : glass is in contact with the plaster but does not mix with it such as it is also to be seen on Fig. 9. Consequently, there is no skin to remove as usual with high-temperature lead-crystal.


Fig. 11 : Detail of the feathers of Salomé’s sculpture “Grand Duc” (N°S-2002-300) : according to the evidence, there is no way to polish this rubbed skin that produces nevertheless a strong effect with this alternance of dark an light ; photo Frédéric Morin

The lighting getting through the glass is also producing the scope of greys rising between white and black, for the print of each tooth of a knife, in a way that polish up the relief (Fig. 10). Such effects were introduced by the cut and polishing of lead-crystal (what for a cost !) but never reached previously with pâte-de-crystal and its common mechanical polishing or acid softening.

Place is missing to develop in what our process differs from tradition that everyone knows. Anyhow, we may point out several specific results :
— First our process ensures that it is possible to employ lead-free glass instead of lead-crystal for kiln-casting at temperatures suitable for normal moulding plaster. Usually, artists using lead-crystal rise 880°C as a minimum for the process, sometimes much more to obtain a very low viscosity (we mean liquid). This is much higher than the blowing temperature with lead-crystal, and a similar viscosity would need 1.200°C as a minimum with glass : there is no more plaster standing at such a high temperature. This first point about the use of lead-free glass opens onto other advantages in addition to safety : the lead oxide of the lead-crystal forbids the use of reducing oxide of copper and turns all of the reducing colours to black. Employing glass, we are able to produce sumptuous reds, both strong, transparent and luminous, some 10cm thick : Frédéric Morin’s sculpture entitled “Red Isis” (N°F-2001-116, h=58cm, 20,2kg, now to be seen at the Musée du Verre in Sars-Poterie) offers a good example of what has never given to be seen before (Fig. 13). Our oranges and yellows are brightening with an unusual force with glass.
— Second, the medium viscosity of the glass forbids it to wet the plaster : glass is in contact with the plaster but does not mix with it (Fig. 10). Consequently, there is no skin to remove as usual with high-temperature lead-crystal, no mechanical nor chemical cleaning and/or polishing. That saves money and health. This second point about the medium viscosity points out another major consequence. As far as there is no need of polishing nor carving such as it stands with Bronze, then the print of glass stands as a perfect reproduction of the original sculpture out of clay created by the sculptor (Fig. 10). This allows the sculptor to play with details of clay such as a rubbed surface left by a wooden tool, either with the regular alternance of darkness provided by the glass-print of the teeth of a knife on the clay. A softened clay will provide a satin-skinned glass... and a temperature limited to 736°C will increase the viscosity in a way that the glass will not be liquid enough to enter the thinnest details of the plaster. So the glass will stay brightening for it had no contact ! The mastery of a lost-clay process that Salomé rised up in our studio brought a major contribution to the expressive sculpture we are producing.
— Third, the low temperature process under the crystallisation point allows to cool the works as slow as possible : we mean some 4°C per hour to rise the annealing temperature. We are far from those who open their kiln to cool down ! During this cooling, a 20kg sculpture will lose a volume measured as one litre (Fig. 11), and this loss has to be fed by extra charge of glass. To allow this feeding, the temperature must remain high enough... and that is the reason why we never open the kiln at that moment.

Our process brings a lot : there is no more local retraction (usually located on buttocks and breasts...) and the plaster has no reason to break. Thus it may stay as fragile, light and thin as possible (not less than 8mm!). Hence this plaster may be used and reused a great number of times, simply adding 20% of new after grinding. This third point makes so that, according to the evidence, our low temperature process limits us into a strong sculpture : we have to deal with shape effects and viscosity. We cannot produce slim items such as «biscuits de Sèvres». On the other hand, there is no limit of size but what we are able to move and the dimensions of the kiln. Our larger kiln is 50cm wide, 1,55m long and 1,20m high (Fig. 7) : we are among the few people in the world who are able to produce 60kg sculptures without any break.


Fig. 12 : Portrait of Claude Morin by Frédéric Morin & Salomé (N°FS-2003-396, h=26cm 12,7kg). Claude Morin (b. 1932) started the Studio-Glass Movement in Europe while creating his personal studio in 1970, in Dieulefit (France). As far as no further carving or polishing is required after our low-temperature process, the portrait of pâte-de-verre is one of the numerous new directions opened by the discoveries of the authors ; photo Frédéric Morin

— Fourth, there is no thermic movement into the mould after casting : the glass felt down from the upper flower pot out of clay and it will stand like that. This allows to employ colour inside the sculpture in different ways of use, opening a range of meaning much more interesting than the common mixed and soft colours which are ordinarily available. We are able to strengthen a body with an inside column of colour, either to distinguish two bodies of a dancing couple with two different colours or to bring life into the thick glass with dancing flames of colours. This fourth point allowed us to make a new observation. Everybody knows that a mix of blue and yellow pigments produces a green coloration, and that a yellow light and a blue light will cover each other to produce a red, no one have ever seen and described the transformation of a white light passing through our colours AltGold and DunkelBlau from Kugler (Germany), such as it is to be seen on a picture of a couple Salomé made (“Tout-Contre”, N°S-2000-67, h=56cm 20kg) : the resulting lighting is purple, as well as it is to be seen from the two lenses one yellow the other blue, or the little bowl blown with two colours we showed during the Congress. Where colours are soft, the crossing lighting appears as green, where the colours are strong the crossing lighting appears as red. This opens wide a new and virgin field for Science.

Referring to Science, our experience of low-temperature process will lead some of the archaeologists specialised into glass to introduce this new knowledge for the interpretation of remains and restitution of processes. The temperature of 750°C is easy to reach with wood even in a direct-flame kiln, whatever may be the period and the technology available : that means that the process may stand long enough at this temperature. Is it possible to imagine a low-temperature blowing, or rather a high-temperature blowing but a low-temperature decoration, in a way similar to lamp-work ? This may explain the virtuosity of the past blowers...
As another scientific result, we brought the evidence that the different constituent elements of glass may be affected by movements under the temperature of crystallisation : atoms remain mobile for the glass is still in the state of a liquid. That brings a new lighting on the necessity of a quick cooling to obtain glass instead of crystallised quartz after a high temperature cooking, such as illustrated by Dr Pascal Richet (Richet, 2000, 105). It must be reminded that variations of temperature in our process are 3°C per hour or 4°C per hour, to be compared to the 1° or 10°C per minute mentioned by Dr Richet between 800°C and 500°C, and that makes a great difference between the two conceptions.
This matter relates also to the interpretation of the process employed at Bet Eli'ezer, Hadera (VIth-VIIth century AD) in Israël where different melting kilns were discovered (Foy and Nenna, 2001, 37-39). The enormous weight of one of the batch recovered, rising 9 tons, is such that it is not possible to imagine that it could be cooled quickly enough, even over flown with water : anyhow the batch would have been recovered broken. Onto the contrary, one of these batches was recovered in one, 3,80x1,95x0,45m at Bet She'arim (Islamic dating ?) and the remains discovered at Bet Eli'ezer show, due to their forms, that they have been broken by dynamic shocks and not by retraction. Based onto heavy sculptures exceeding 60kg, our low-temperature process contradicts the present state of knowledge based onto scientifically experimentations made on small quantities, reaching sometimes 5 gr. .

As a conclusion, we consider that technique is nothing if not employed to express feelings... As far as we are Artists, we act as a sponge, we mean not of alcohol but of human sentiments. We feed our work with the humanism that brings a careful attention of what happens everyday, attention to what life is made of, most often in a general perception rather than individual behaviour.
Opposite in that to the laws of fashion, we give body to those feelings we are afraid that they may go missing, that is tenderness, gentleness and sweetness, generosity, happiness, a caressing pride and strength, deference, including the right to be different, the interest to bring complementary...


Fig. 13 : Frédéric Morin’s sculpture entitled “Isis” (N°F-2001-177, h=58cm, 21kg) offers a good example of sumptuous reds, both strong, transparent and luminous, although some 10cm thick ; ektachrome Bernard Coste

Our caressing sculpture is regarded as a gift by those who know a little about life. And our strongly coloured transparent glass sculpture, luminous due to this low-temperature process, enlightens our everyday life.

We would not like to end this paper without special thanks to Joseph Philippe, Giuseppe Cappa and Ger Maas who delivered so much efforts to support our work, and also to Lucien Wercollier and François Wagner who trusted in our abilities and stand in favour of our enterprise.

Frédéric MORIN and SALOME


BIBLIOGRAPHY
Cappa, G., 1998, Le génie verrier de l’Europe, Liège, Mardaga, pp. 342-343 et 540-541
Cappa, G., 1999, ‘Lucien Wercollier et l’art verrier’, in Les Cahiers Luxembourgeois 3-1999, I-XVI
Cappa, G., 2001, ‘Postface’, in Morin, F. and Salomé, 2001, Verre en Forme, Montoison, Verre en Forme, pp. 30-31
Foy, D., Nenna, M.-D., 2001, Tout feu tout sable, mille ans de verre antique dans le Midi de la France, Musées de Marseille / Edisud, Aix-en-Provence, pp. 37-39
Kermer, W. and Kermer, F., 1993, Claude Morin, Verrier de Dieulefit, Glasgestalter aus Frankreich, Stuttgart, Arnoldsche
Morin, F. and Salomé, 2001, Verre en Forme, Montoison, Verre en Forme
Philippe, J., 1995, Sculptures contemporaines en cristal et en verre, Liège & Luxembourg, Liège, Fortemps SA, pp. 239-240
Philippe J., 2000, Sculptures contemporaines en cristal et en verre, catalogue de l'exposition Liège-Luxembourg, Liège
Richet, P., 2000, L'Age du Verre, Découverte Gallimard / Technique, p.105
Morin F. + Salomé,  2003, Lost-Clay Moulding and Low-Temperature Kiln Casting : Two Revolutions for Pâte-de-Verre, in Annales du 16ème Congrès A.I.H.V., London, pp. 308-313