Silver Foil Inclusions

Silver foil is better for inclusions than silver leaf as there is more substance and so less likelihood that it will burn away.

Sterling silver – not that common in foils – will darken easily on exposure to temperature and air.

Silver foil will also react with some glass colours. A good guide to this can be obtained from Bullseye’s chart on glass interactions. This shows which glasses react with silver. To minimize the reactions, minimize the amount of time spent above 600C.

Only a short bubble squeeze is required with foil as it is not so stiff that it will resist the weight of the glass on top, so creating bubbles. A short squeeze to allow the air out is still a good idea.

Venting moulds

Raising the mould from the shelf to provide ways for the air to get out of the mould is as important as providing holes in the mould itself. Often people recommend placing the mould on kiln furniture, but it is very easy to have a number of pieces fibre paper to stack in three places under the edge of the mould to provide space for the air to be expelled from the kiln by the dropping glass. Only a little space is required for the air to escape.

Vertical Kiln Formed Holes

For vertical holes in frit-cast reliefs you can fill a drinking straw with plaster, cut while still wet and build the frit or cullet around it.

Already fused pieces can have the holes made much neater and smoother by using the above method in pre drilled holes. Another method is to wrap a thin strip of fibre paper around a pencil or end of a paint brush. Then push this circle of fibre paper into the hole. If this is the same height or a little less than the glass, it provides a clean fire polished hole, if the glass is taken to the high end of fire polishing temperatures.

Found Moulds

Found moulds are often ceramic bisque or greenware, sometimes glazed and fired for house or other final use. Many other materials, usually metals can also act as moulds. This article will address ceramic materials.

Shape

Pick out a mold that is not too complicated, detailed or deep. A shallow bowl or plate with a rim is ideal. When choosing bisque ware to use as a slumping mold, avoid complicated and deep shapes. Do not choose molds with intricate carvings or patterns for slumping. Those shapes would be better for frit casting. Instead, choose shapes with a rim or with gentle curves rather than steep slopes.

Slumping or Draping

Glass has a higher expansion and contraction rate than ceramics. This means that any draping has to be done over gently curved ceramic materials. So the general advice is to avoid draping over ceramics. If you do drape anyway, it is advisable to cover the ceramic with fibre paper in addition to the kiln wash.

Vent holes

You need to drill holes in the proposed mould to allow the air to escape as the glass slumps. You might want to see what holes are drilled on similar plates online. The holes should be small – about 1.5mm. Much smaller and they will get clogged up with kiln wash; much larger and they will mark the glass. The drilling should be from the inside to avoid any break out into the moulding surface.

Greenware is easy to drill, so don’t press hard; let the drill bit do the work.

Ceramic forms that have been glazed require more care to start the hole. The surface is so smooth the drill bit will tend to skitter around. You can place a bit of tape where you want to drill to reduce the movement of the drill. You can also start the hole by using a masonry drill bit and rotate it by hand at the point you want to drill. This will create a “divot” in the glaze to hold your drill. It is also easier to drill, if you sandblast the glazed surface first. This will give a bit of “tooth” for the bit to grip.

You should drill the hole(s) at the last place the glass will fall. In a completely round bottom you drill at the centre. If there is a right angle or steep part near the bottom of the form, that is the last place the glass will touch and so is where the holes should be drilled. Three, spaced equally apart, should be enough.

Preparation for use

Take the greenware and clean it with a mild abrasive pad or nylons to eliminate the mold marks and scratches on the piece.  Have any greenware fired to at least bisque temperatures at a ceramics studio. Explain what you are doing and the working temperature. The ceramic does need to be fired high enough to be robust.  The ceramics people can give you information on the performance of the ceramic when fired to various temperatures.  The bisque mold must be kiln washed before use.

If you are using an already glazed form, you need to remove or roughen up the glaze enough to take the kiln wash. A sandblaster does a good and quick job, but it can be done by hand with wet and dry sandpapers. The process should be done wet to keep any dust from the glaze (a vitreous power) getting into your lungs

Test
 
Finally, you should test your mould with glass that has little value, before committing you best efforts to the mould.

Thick Glass Firing

by Tony Roberts

My schedule for 50mm (2 inch) Pilkington’s Opticlear in a top-heated flatbed kiln is:

0 to 600C - 6C per hour rise - takes 6 days (if you start with a solid slab) (I start with smaller pieces, so can raise the temp much faster)

600C/hr to your soak temp - as fast as you like (I go to 840C and hold 4hours)

Soak temp to 565C - drop as fast as you can, then hold for 14hours

Anneal: drop at 0.75C per hour to 365C - this takes 11 days

Then drop at 1.5C per hour to 300C - another 2 days

Then drop to 60C at 4C per hour - 2 days and a half



A total of 16 and a half days

Glass Selection

I have produced some notes on some of the elements in the selection of glass.

Here are the links:

Glass Density
Clarity
Advancing and Receeding Colours
Light and Dark
Colour Combinations

Choosing Glass for a Harmonious Appearance, 3

Different colours, of course, have different appearances. The most commonly known one is the hot/cool colour combinations. This still applies when dealing with opalescent glasses, where reflection is the dominant experience of the colour.

But in glass where there is quite a bit of light transmission, the receding and advancing colours are not exactly the same as in painting and opalescent glass. The greatest separation comes with intense red (close) and intense blue (distant). In some circumstances these can be experienced as apparently being in different planes.

There are a few distinct advancing and receding colours, but most are much more subtle and are not all as expected from the experience of reflected colour. Clear, for example appears nearer than a strong blue. It is up to each person as to how far they wish to take these combinations.

Those who do want to investigate, should go to a place where they can view windows with small pieces and a variety of colour in strong light. They can then record which colours appear to “float” above others, or recede.

Choosing Glass for a Harmonious Appearance, 2

Clarity of colour

When considering the representation of distance or depth you need to look for glass that is less pure. The colours that are muted or have a touch of white, blue or grey will provide a good representation of distance. The pure colours will appear more brilliant among the more muted colours.

This is where glass samples can be most useful. By holding them up to the light, you can see the effects one glass has on another and how one colour will appear among the others.