Home Made Billets

You can make your own billets from small pot melts.  But why should anyone go to the effort? Some reasons are:

• ·        You can make your own colour. 
• ·        You can use your cullet/scrap (avoiding buying or making frit).
• ·        You don’t have to buy and break billet to size 
• ·        You can reduce the clouding caused by many microscopic bubbles surrounding the frit pieces. 
• ·        You can make a size to fit your casting mould. 
• ·        Potentially, you will reduce needling.

 Now you are convinced of the advantages, you want to know how.

Preparation

• ·        Select the glass. Avoid iridised glass and any ground edges – they will cause haze in the final casting. Wash all the glass. Place the glass in a small flowerpot.
• ·        Weigh out the amount of glass cullet needed for the mould and add about 50gms to account for the glass that will stick to the pot.  Calculating the required weight is relatively simple and this post gives the information.

Dams

• ·        Arrange dams in such a way that the resulting billet will fit into the mould without overhang.  It might be quite a tall billet. In which case cast it horizontal with the height as the length of the billet.
• ·        Line the dams with Thinfire/Papyros at least. One mm fibre paper would be better. 
• ·        The dams can be on a kiln washed shelf or on fibre paper. The bottom of the glass will be fine either way.
• ·        Place the pot above the dams.  The higher, the fewer bubbles in the billet.  And any left in the billet will be reduced by flow in the casting firing.
• ·        Multiple billets can be made of different colours, sizes, etc., at the same time.

Firing

• ·        Fire to around 900ºC/1650ºF and soak for hours.  Observation will show when the pot is empty.  Clue: There will be no string of glass from the bottom of the pot.
• ·        Anneal as for the smallest dimension.  If you are doing multiple sizes, the dimension must be taken from the biggest piece.
• ·        When cool, remove and clean the separator off the pieces thoroughly.  A 15 minute soak in a 5% citric acid solution will speed the process.

Casting

• ·        Place billet in casting mould. The first ramp rate needs to be for the smallest dimension of the billet.  This may be a slower rate than when using frit for casting.
• ·        Do a long bubble squeeze in the 650ºC to 670ºC range – up to two hours, but a minimum of one.
• ·        Fire to your normal top temperature and time.
• ·        Anneal for the largest piece.

 

More information here 

Pot Melts – Weight of Glass Required

Circular pieces
This table assumes that a 150 mm diameter pot is being used, and assumes that 125 grams of glass will be left in the pot. Larger diameter pots or even pot trays can be used, but more glass will remain in the container. The following table gives the desired diameter of the melt and the weight of glass needed to achieve an average 6 mm thick result. To achieve a uniform six millimetre thick disk will require long soaks at both melting and fusing temperatures to allow the glass to even out in thickness.

50 mm diameter disk requires 154 grams of glass
100 mm diameter disk requires 243 grams of glass
150 mm diameter disk requires 390 grams of glass
200 mm diameter disk requires 596 grams of glass
250 mm diameter disk requires 861 grams of glass
300 mm diameter disk requires 1185 grams of glass
350 mm diameter disk requires 1568 grams of glass
400 mm diameter disk requires 2015 grams of glass

Thicker melts
Of course if you want a thicker pot melt — one that is confined so that it cannot grow larger, only thicker — you can use the following method to estimate the amount of glass required.

Choose the diameter wanted from the above table, and subtract 125 from the weight of glass required. Then multiply by thickness wanted divided by 6 mm. Add back 125 gms — the amount that will be retained in the pot — and you have the required amount.

For example: a 200 mm disk of 6 mm requires 596 gms. You want a 12 mm thick disk of 200 mm.
First subtract 125 from 596 to get 471 gms. 417 gms times 12 equals 5652. Divide this by 6 mm and you have 942 gms required. Add 125 gms — the amount left in the pot — and you have a requirement of 1067 gms for a 12 mm thick disk of 200 mm.


Rectangular pieces
These are easier to calculate than discs, as the calculation is length times height times depth (all measurements in centimetres).  

If you are making a billet and using an empty margarine pot of 7 cm wide, 12 cm long and 7 cm high you will need enough glass to fill a volume of 588 cubic centimetres.  As the specific gravity of glass is 2.5, you multiply the cubic centimetres to give the weight required in grams — in this case, 1470 gms.

If you wanted a 6 mm tile of 100 mm square you would need 150 grams of glass.

To make a 1 cm slab of the same size you need 250 grams of glass.

To make a billet of 5 cm by 10 cm square you need 1250 grams of glass (this is pretty close the the maximum that can be loaded in a 12 cm diameter Pot).

To make a small sample billet of 2 cm thick by 4 cm by 8 cm you need 160 grams of glass.

A billet or pattern bar of 5 cm by 10 cm by 5 cm needs 625 grams of glass.