How to make a basic cast-glass object
Silicone rubber moulds: decorative, textural objects
Ahead of time prepare embellishments with which to decorate the main form. Decorations can be anything that appeals. Organic materials work well — leaves, flowers, bark, seeds, or materials such as lace, buttons, jewellery, etc.
To avoid sticking, and depending on the material, apply either spray-on polyurethane, which protects porous objects and stiffens soft, delicate materials such as lace, or Vaseline, applied with a paintbrush or your fingers. This stops the silicone sticking to metals, plastic, and glass. Detergent solution can be used to keep fabric or paper materials moist.
Using silicone rubber
Apply silicone rubber (e.g., Ados RTV Acetic Cure silicone from Mitre 10) to your object’s surface with a caulking gun. “I do it on magazine glossies like New Zealand Woman’s Weekly, because it peels off easily,” Carmen says. Pipe the silicone on in overlapping runs to avoid air bubbles. You can also squirt it into soapy water (which stops it sticking), take it out with your fingers, and apply. Alternatively, you can wet your fingers with detergent and water, then spread the silicone gently and evenly over the textured surface. Take care not to trap air under the surface. Make the silicone rubber mould 1cm thick.
Leave 12–24 hours to cure.
Once cured the silicone can be peeled off the form. Clean the interior with soapy water if needed. It is now ready to be used, many times. The next process will use wax.
Making wax moulds for lost-wax casting: Nick’s shot glasses
Use microcrystalline modelling waxes to make the form. These waxes are easy to work and burn out cleanly at a low temperature without catching fire. Nick made three shot glasses. Here is the process he followed:
1. Melt wax in a crockpot (you won’t be cooking stew in it again) to a temperature of around 80°C.
2. Fill two small balloons with water — water-bomb style — to the size you want your form to be. One will be the base and the other the top.
3. Dip around two-thirds of the balloon in wax. Remove and allow the wax to harden and cool down — you can
also dip it in water. Repeat the process about seven times to create a ‘slip’, or ‘shell’.
4. Pop the balloon with a Stanley knife or pull it out.
5. Join the two balloon shapes at the apex of the curve to make top and bottom, and anchor with wooden skewers or toothpicks, which will burn away in the kiln. Then add more wax (a thin sausage) around the middle and smooth it out using small metal instruments, such as a spoon or knife, ground to a shape that suits you, and heat over an alcohol burner flame.
6. Add on any premade decorations. (Nick used berry moulds on one and, on another, a latticework of flax on the outside with a flower shape in the bottom of the glass.)
The goblet forms can be sculptural pieces or functional.
When the wax mould is finished, make, or ‘invest it with’ the refractory mould. Make the refractory mix with a 1:1:1 ratio of water (to mix), plaster (to hold the form), and silica (to stop it breaking up in the kiln).
Use clean water and allow the plaster to ‘slake’ for two–three minutes before mixing. Gently stir without making air bubbles. The mix is like whipped cream. Next, with your hands, mould the plaster around the piece. Do lots of layers — in the kiln, if one layer goes, the next will hold it. The internal layer around the wax is the important one because it catches all the textures, so it needs to be sound. These are the ‘core moulds’ and they have an internal and external surface. The base, or ‘sprue’, which is the other cup form, is solid. That is where the glass will go in through a feed. The glass pieces sit in the top and melt down into the sprue then in and around the bottom cup form. Allow the refractory to cure for 24 hours to reach maximum strength.
Losing the wax, or ‘cire perdue’
Make sure the steamer heats up the mould from bottom to top, so that it is too hot to touch. Steam out the wax. “You don’t want any wax left in it as it makes marks on the glass,” Carmen cautions. “We want a clean surface. The wallpaper steamer is gentle.”
The process begins with a positive, goes to a negative, then back to a positive. “Ideally once the mould has been steamed out, leave it long enough to dry (a week or more) before putting it in the kiln,” she says. “Moisture from the mould can rust the kiln. But I don’t have time when running the weekend workshops.”
Work out how much glass you will need. Carmen uses the water-displacement method: First mark the water level on a jug. Pour water into the mould up to the level you want the glass. Next, tip the water out of the mould and see the difference between the mark on the jug and the water level, then put glass into the jug to make it up to that level.
The firing process is very technical and has to be followed step by step according to a schedule. If you don’t have a kiln, Carmen reckons sheddies could do
90 per cent of the glass-casting process themselves and take the object to a kiln for firing — you can take your objects to a facility that hires out kilns, or you could ask a glass artist to do it for you. Carmen heats up the kiln slowly to let the mould dry out, then holds it at 800°C to let the glass run in and the air come out. During the firing process, the glass melts down through the hole in the base of flower-pot reservoir — which is placed on top of blocks sitting on top of the plaster mould — into the mould, filling the empty cavity left by the wax. “Hold for a few hours then crash the kiln, cooling it fast to stop the glass flowing. Then crash it down again to the annealing temperature during which all the particles of glass slow down and lock into position so you don’t get any cracks,” she says. “If you don’t anneal properly, you’ll get cracks in the glass. Hold at annealing temperature for the recommended time of the glass manufacturer, which lets the mould and the glass cool at the same time. Then cool through several stress points until it reaches room temperature. It can take days or weeks. You need to work it out logically according to how thick the glass is.” The workshop students’ objects are usually ready after four days.
“I try out lots of different things. I put on my firefighter’s outfit and I’m in there with my gloves at 800°C putting glass in and taking pots out with different reservoirs if I’m trying to mix glass in a certain way,” Carmen says. “There are lots of things you can do when glass is molten as long as the refractory mould is good with no cracks. It’s quite fun but you can’t be scared of heat.”
Removing the mould
Take the mould off carefully. Be gentle; pick away with a blunt peeling knife, always mindful of where the glass sits. “Pretend you are an archaeologist, and don’t chuck it in water because the plaster can expand and crack glass,” she says.
Use a toothpick and toothbrush to clean away the plaster.
Carmen uses various machines to refine the glass object and “sort out any problems”. The wet-belt linisher does soft edges and the diamond flatbed levels the flat base of the shot glasses. Nick smoothed off rough bits around the rim using the Dremel and then air-dried his shot glasses before avsandblasting. The sandblaster creates tiny, even pits in the surface of the glass with a mixture of air and garnet. Once that is done the glass can be put into acid, which eats into the fine holes. “Sometimes I use sugar acid, which gives a durable satin finish,” says Carmen. “Otherwise I squirt the piece with silicone once it is sandblasted. This protects and gives a nice sheen. You have to wear long gloves and a mask when working with sugar acid, and be fully covered as it is quite toxic.”
Silicone is spread onto an object to create a mould, then wax is poured over the mould to create an impression. This is then attached to a larger object using a warm implement to refine the wax
Dipping a water-filled balloon into wax to create a form
Applying the first coat of plaster using a paintbrush to ensure all the detailing is picked up. A mask guards against toxic fumesBuilding plaster around the object, making sure it is evenly coated
Many colours of glass are available from suppliers
The balloon is popped and extracted before adding details
Steaming the wax out of the moulds to create the cavity for the glass to flow into
Removing the blocks from the hot kiln
Small pieces of plaster are removed from the glass in the sandblasting machine