The Glass Casting Process

Casting Glass is not for the faint hearted, it is expensive to establish a glass casting studio and time consuming. If you can rely on a school to get you started or a mate with the right gear, do this first. When you think you have spent enough money, get ready to start buying more equipment and or glass.

It took me 3 years full time to become content with my skills base. I am now comfortable about casting glass with a predictable out come (fingers crossed). It does help if someone can teach you.

This is a basic run down on the procedures from start to finish there is approx. 25 separate steps in the process which does not include finishing.

- Design the work 1 - 200 plus hrs. depending of course how intricate or large.
- Mold any piece that may be reproduced in Silicone Rubber. This should be seen as insurance against damage or loss in firing.
- Cast or build the work in Wax, and finish for investment and firing.
- Design wax Sprues and Gates for getting the glass into the mold and venting the air out. Best to design your air vents in the upside down position so that you can visualise how any air may become trapped.
- Make molding box to invest the work in plaster allow at least 30 mm or 1" around the sides for molding strength, also determined by the amount of glass to be cast. I use a thin gauge galvanised steel as its generally easy to conform to the desired size. I seal the bottom (perspex) to the tin with wax applying the wax (or welding) with a soldering iron.
- Mix up investment material ready for casting a typical mix is 50% casting plaster to 50% (by weight) of 200 - 400 grade Silica Flour (beware as this is a carcinogenic material, use all precautions) also add some shredded fiberglass strands cut to around 10 - 20 mm in length for greater mold strength. This is a basic mold mix, there are plenty of ways to change the mix for different applications.
- Pour mixed material into mold box from the sides so that investment does not pour directly onto sculpture, creating more air bubbles. I pour on a vibrating table so that the vibration shakes the bubbles away from the piece and it spreads from the bottom up, pushing the bubbles as it runs in. Insure that the object being cast is well secured to the base or it will come off.
- Allow to cure for at least 8 hrs.
- Remove from casting box and clean up the plaster block, I have found that a line traveling up the side of the mold can create a path for a crack to start. I scrap all external lines off the mold so that it is flush.
- De-wax the mold either in a burn out kiln or via steam. I use the steam method as it seems much more passive on the mold. I use a stainless steel laundry sink over a gas flame and completely cover the top to trap the steam. This can take 1 - 4 hrs depending on size or complexity of the work. Wear rubber gloves when handling the mold, they don't seem to scald or burn as quickly as skin. Eye ware is also advisable as the mold could slip out of your hand back into the boiling water and wax and splash into your face. (Not Good) This is of course if the work is not to large to handle.
- Pour boiling water into the De-waxed cavity to clean out any remaining residues of wax, the remaining wax residue will burn out in the kiln.
- Once I have rinsed the cavities I use a graduated cylinder and measure of hot water to determine the volume of glass required to fill the void. This must be poured in quickly as the plaster will continue to suck up water and of course throw out the estimate. Be careful here to be as accurate as possible so that hopefully you will have just enough glass to fill the void. One can also use the water displacement method of measuring an object. Remember your gates and sprues will take up glass.
- Dry the investment out as thoroughly as possible before placement in the kiln so as not to create to much steam in your kiln (electric) as it may cause a short in the wiring. Placing it in a covered box over a LPG (Propane Burner) on a low heat, generally 4- 6 hrs. for medium sized work will suffice. Or even the kitchen oven if your partner or family can stand the smell. Be Aware of the ventilation around your kiln as it comes up to 400C it will start burning the wax. Do not do this in an enclosed area the vapor's given off will be toxic to your health.
- Place any crucible stands in place on the invested mold after placing in the kiln. You do not want to be stuffing around with the kiln once it is up to temperature (820 - 860) as the chilled air entering the kiln will start cracking the plaster mix rapidly and potentially introduce hair line cracks in the investment.
- Take the invested mold up to 300 C as quickly as you like and allow it to sit at this temp for about 3 - 4 hrs or till you think it can go up higher (its a feeling one gets) once you smell a faint burning wax smell it means that the internal temp. of the mold is at around this mark.
- Depending on my level of confidence about how dry the mold might be I either take it up to 600C or 840C immediately. I have found that a freshly invested and force dried mold seems to be stronger than one that has been sitting around for a week, but don't quote me on this.
- If you are casting a large object with a lot of glass it may be a good precaution to wrap Nichrome wire around the outside of the investment, in case the mold splits. ( Electric Heating elements are a source of Nichrome Wire) This can be tricky because the investment will shrink as it dries further, leaving your previously taut wire hanging a bit limp. I have found that preparing the wire for a second "twitch" to draw it tighter at around the 500 - 600C mark works. This is dangerous and I don't not recommend you try it, so be it on your behalf. Forward planning is essential, leaving a small amount of wire to twitch with long nosed pliers ( the longer the better) at the most convenient corner towards you. Wearing kiln mitts and long sleeves is essential. If you are using a top loading kiln - forget it, only a front loading kiln will be accessible. If you do this be quick, twitch one, if you hear any (clinking sound) close the door and wait for the kiln to come back up to temperature, then do it again. Do not allow the plaster to cool to much as the thermal shock is immediate.
- Prepare crucibles. Terra Cotta flour pots work just fine, you will have to open the hole up more depending upon the amount of glass flow you may require. Sand the holes edges with wet and dry paper under water. I also use a scotch bright pad to scour the internal and external bottom of the flour pot so that no pot residue exists. Cleanliness here is an issue.
- Measure out your glass requirements ready for insertion. If the glass has a number of colours measure them into different piles depending on insertion periods. Wash glass thoroughly in warm soapy water and rinse well. I preheat my glass in a separate oven to dry the glass and have it waiting for me at maximum oven temperature, around 250C. This also helps to save your kiln from glass exploding in the kiln if it is added at say 840C and showering fragments around the kiln that will stick to the walls if it hits them. Preheating the glass also shortens the casting period as it doesn't need that extra time to come up to temp.
- Depending on how much of a "glass well" you may have on the top of your pour hole or how wide your glass delivery sprue is determines how much glass one should place in your crucible. Glass will of course be restricted by the size of the hole which can be an advantage when a slow delivery is required or a huge disadvantage if you have lots of glass to get into the mold.
- Once all the glass has been delivered into the mold I allow at least 10 to 12 hrs. at casting temp for air bubbles to rise and hopefully dissipate. This is a good time to be in bed, yet if your mold determines that it wants to crack and leak glass into your kiln then sleeping through this event can be a bummer.
- If you are happy with the amount of air bubbles that may be evident in small quantities on the surface of the pour holes it may now be time to start cooling the kiln down.
- Crashing a kiln is a term for rapidly cooling the kiln to under 700C. Yet this is also a problematic method of cooling as the investment wants to crack very rapidly which can lead to fissures opening internally and glass leaking into the cracks. Some glass can (devitrify) which is a crystalised growth that inhabits the surface and internals of the glass this can migrate into the body of the glass. Hence using lead crystal which tends to be able to go through this cooling phase and maintain clarity without extreme crashing. This is an area that the artist must experiment with. I do not crash my kiln now as much as when I started and find that it just takes longer to cool the kiln down, yet now I have little to no internal cracking.
- Annealing the glass is probably the most important period. Your glass supplier will give you all the relevant data on glass annealing schedules. Schedules are based on thickness of glass and allowing the internal strain to modify and dissipate evenly throughout over the cooling period.
- Allow the mold to cool almost completely before removing from the kiln, rapid changes in temperature can crack exposed faces in your glass sculpture.
- Once removed allow to cool for a further period before cracking off the investment as there will be a residual heat within the centre of the mold.
- Always wear a dust mask that is rated for silica when working with the investment, work safe. Silica Flour must be treated with respect it is a known to be carcinogenic.
- Allow the glass to stand at room temperature for a further period of time before washing any remaining investment from the work. Be very Patient.
- Due not use hot or cold water to clean the piece as you may get a great surprise if the work develops a crack, estimate the air temperature and regulate your water temp to this. Better still wait till tomorrow.
- Cutting sprues and risers is only done with a diamond rock or glass saw, do not use a diamond tile saw as there are many cheaper diamond blades in hardware stores now. Use only water or a suitable coolant to cool lapidary cuttting tools.
- I won't go into the finishing and polishing as this could take up another few pages and a couple of years to learn, if you have got this far your bound to develop finishing styles.

Disclaimer: The methods explained on this page do not guarantee any success in any manner. The author excepts no responsibility what so ever for any accident of failure due to anyone's use of these procedure's. This is only intended as a helpful insight into the process and is to be considered a very broad overview. You have to have a go!