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.

Choosing Glass for a Harmonious Appearance - 1

There are at least two major elements in choosing glass: density and clarity. A third is the “hot/cool” effect of colours. The appropriate combination of these elements leads to a panel with bright or hot spots where you want them. You can create a dramatic image or a more restrained one with more gradual gradations of light without obvious bright or dark areas.

Density

Density relates to the amount of light the glass allows through. Clearly black is the most dense glass – allowing no light through. In general, glass can be divided into opalescent and transparent.

Opalescent glasses range from the very dense opaque to less dense translucent glass.

Transparent glasses have a variety of densities too, although almost always less dense than opalescent glass. The density of transparent glasses relates to the intensity of the colour and the texture of the glass.


Colour intensity

The intensity of the colour is related to the amount of light allowed through. The intensity is directly related to the saturation of the colour. A further effect on colour intensity is the thickness of the glass. If you look at a handmade sheet of glass with different thickness on one end to another end, you can see the gradation of the colour and the amount of light that comes through.

Glass Textures

The texture of the glass affects the density of the glass. A smooth glass will have less density as the light passes through without dispersion. As the glass becomes more textured, the light is more dispersed and so appears more dense.

Glass Breaking While Soldering

Some report breaking pieces of glass while soldering. This may happen more on pieces that have big differences in width or taper to thin points. What is happening is that the glass is being heated too much locally in relation to the rest of the piece.

The solution is to solder at a steady pace. This allows the solder to cool without transferring so much heat to the glass as to break it. Some recommend that you do not rest your soldering iron on the foil while soldering. However it is the solder which is the heat sink, so the effort of holding the iron above the foil is not really necessary if you move at a reasonable pace.

This means that you do not stop with the iron on the seam. It is best to solder in one continuous movement along the seam, leaving an even bead behind. Sometimes the bead is not even. This may be because of wider parts to the seam, or inadequate flux, or many other reasons. Do not try to repair this before going on to the rest of the seam as this builds up heat in the adjoining glass. Since glass cannot dissipate heat well, the glass breaks when the temperature differential between the hot and cold parts of the glass is too great. Instead, complete the soldering of the seam before coming back to it. This gives you time to decide why the bead is not as good as you want it to be. It also gives time for the heat to reduce and even out through the piece of glass.

As you become experienced you will find a pace that suits the kind of bead on the joint that you want to achieve. If the seam is too flat, slow your pace or increase the rate at which add the solder to the iron. If the seam has too big a bead, increase your pace or reduce the rate at which you feed the solder. It is also possible to consider other methods of soldering.

You also need consider the usual problems relating to cleanliness and insufficient flux. Sometimes the soldering iron is not hot enough, but you should notice this early as the solder will not be melting at its usual rate and will be grainy in appearance.

Temperature Rise Rates

I am always concerned when people recommend soaks on the way up in order to equalise temperatures. If the soak is required because the ramp rate is too fast, there are breakages going to happen sometime - maybe not now, maybe not tomorrow, but certainly sometime. If you need that extra time, add it into the schedule. E.g., a ramp rate of 200C from 20C to 520C with a 20 min soak could also be written as 176C/hr from 20C to 520C - both take 2.833 hours to achieve the same temperature. A controlled heating rate is preferable to one or more rapid rates with soaks.

I am also concerned about very rapid temperature rises after the bubble squeeze. The controllers often cannot adequately control such rapid rises. The rapid rise also often requires a higher target temperature to achieve the desired effect. This can mean that it is easier for bubbles - large and small - to form and rise to the surface during the overshoot of the target temperature. Temperature increases are about heat work - the combination of temperature and time. This means that you can achieve the desired result in two ways:

1- fast rise to high temperature or

2- Slow rise to lower temperature.

The second strategy may also require a longer soak at the target temperature than the one with a fast rise to a high temperature.

The aim in kiln work should be to achieve the effect you want at the lowest practical temperature. This is because glasses tend to change their characteristics more at higher temperatures than at lower temperatures.

Dams - Links to Tips

There are a number of tips on damming kiln formed glass scattered around the blog.  This is an attempt to provide links to them.

Damming Options for Ovals
Description of Needling
Prevention of Needling
Damming Options for Irregular Shapes
How High Should the Dams Be?
Separators for Dams