Resin-Strengthened Plaster

Resin-strengthened Plaster

Martin Hunt explains a technique to strengthen plaster models.

In the current Queensberry Hunt exhibition in the Ceramics Department of the Victoria & Albert Museum, London, we show some of the design models, made from plaster of paris, that we make to present our new designs to retailers and to the potters and engineers at the pottery factories. A plaster model cannot demonstrate the full ceramic qualities of the final pot, but it does, we hope, describe the form with precision and confidence, and gets the whole manufacturing process on its way. But we are faced with the problem that models made of plaster are fragile. We need to make them stronger to survive, as these models are often sent around the world to be assessed and handled in endless meetings!

At QH we have developed a technique whereby we impregnate the models with a synthetic resin to strengthen the plaster of paris. This same process can be helpful in other situations, wherever it is handy to harden and strengthen plaster objects (or other porous materials) in the workshop. Plaster working will be familiar to potters who have studied at art college, and at some future point it would be good to write in some depth about the model-making techniques that we use. But for the moment I would like to describe one part of our process – the impregnation of plaster objects with epoxy resin.

The aim is to soak the porous plaster in synthetic resin. The plaster must be dry. When the resin is set the plaster becomes waterproof, impervious, strong, and hard. The resin becomes part of the body of the plaster and, unlike paint, adds no coating to the surface that might obscure detail. The hard plaster/resin surface can, if needed, be further worked. It can be filed, sawn, sanded, polished, painted, and modelled onto with fillers and resin pastes. Production moulds can be made from the final object.

The plaster must be thoroughly dry. Dry plaster often has a ‘skin’ of impervious material that has been drawn to the surface during drying. This dense surface resists the penetration of the resin and so care must be taken in the drying of the plaster so that the skin, if it forms, is on part of the object that can be sanded for its removal to re-expose the porous surface. Cover the more important or delicate surfaces of the plaster with aluminium foil during drying, and leave the less important surfaces exposed to the air.

Alternatively, if all of the surface of the object is important and one can’t allow any of its surface to be abraded to remove the skin after drying, then another process can be used. A thick slurry is made from water and fine grade molochite. This slurry is sloshed thickly with a brush all over the moist plaster object and stiffens to form a coating on the porous surface. The unwanted salts and other materials that would skin the object and make it partially impervious are carried outwards to the surface of the molochite, leaving the plaster below perfectly porous. When totally dry, the molochite can be gently and easily rubbed off the model (mask to be worn), and the final removal of the dust is best done with the soft brushhead of a vacuum cleaner. For speed, the damp molochite covered object can be force dried in front of a fan heater.

The molochite, removed from the plaster object, can be reused many times if washed free of soluble rubbish. Mix it with plenty of water – it sediments out quickly – and pour away the affected water. Note: the molochite sediments out like a rock! So it is useful to make the slurry in a plastic bag (supported in a bowl) or a flexible rubber bowl. Then the slurry can be massaged back into fluidity!

The epoxy resin is of a type that has low viscosity and easily penetrates the porous surface, and it can be more rapidly absorbed if applied to warm plaster. The resin thins as it is applied to the warm surface. We warm the plaster models to about 40°C (not higher) in an old electric oven (definitely not the oven in your kitchen!).

The resin is in two components – resin and a hardener – and it is thoroughly mixed in the ratio of 100 parts of resin to 45 parts of hardener, by volume.

Paint the resin mix onto the warm object (one-inch cheap white-bristle brushes are ideal and disposable). Place the model back into the oven until the resin coating is absorbed, remove from the oven and add further brushed layers of resin. Do this for about one hour by which time many coatings will have been applied. As the absorption slows, continue to paint the still porous areas and, at the end of an hour, before the resin starts to thicken towards its set, wipe off any remaining resin with paper towels, leaving the model dry with no excess resin on the surface. The resin, when it starts to set, sets extremely quickly – so an hour is enough!

Leave the impregnated model in the oven to set at 40°C for at least twelve hours. A little waste resin can be put in the oven to test the set – and any stickiness means that the plaster requires more cooking time.

When modelling something like a teapot, we form, dry, and impregnate the body of the pot and, after impregnation, polish the model with fine wet and dry abrasive paper. The plaster from which the spout and handle are to be modelled is then cast against the teapot body, removed, modelled, and in their turn, impregnated – and then attached permanently. Any ‘changes of mind’ can often be made by the application and modelling of resin paste (white car-body polyester filler). Handles can be made to fit the curve of the pot body with the same material.

Fumes from the warm resin can be irritant. A mask, protective against ‘light organic fumes’ is advised. We use an old electric oven for warming the models with a cheap cooker hood close above the door to evacuate fumes. The extraction hood makes the wearing of the mask less necessary.

Yellow kitchen gloves protect the hands, and goggles the eyes during the wet resin process. Tight-fitting latex gloves are advised for subsequent working of the resin impregnated surfaces. Resin and resin dust can cause skin sensitivity.

Work the plaster/resin surfaces damp, wherever possible, to avoid the build-up of dust, which must not be inhaled. It’s best to continually mop up dust with damp sponge-cloths. Do not rub your eyes!

Wear a good dust mask if at all unsure!

It will be good to hear of the thousand new workshop uses that you will find for this resin!