How to make a ba­sic cast-glass ob­ject

The Shed - - Glassmakin­g -

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.

Re­frac­tory mould

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.

Con­tin­ued ...

Kiln fir­ing

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

Wet-belt lin­isher

Small pieces of plas­ter are re­moved from the glass in the sand­blast­ing ma­chine

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