Which glass to use?

Boats, Maureen Anderson

 Glass: which one to use?

Almost anything which can be called ‘glass’ can be used by the kilnformer. Most of it will be soda lime glass, so it will have a coefficient of expansion (COE) somewhere between about 80 and 100. Any single piece of glass can be bent or slumped, whether it be a bottle, a piece of window glass or a piece of the most expensive coloured certified fusible sheet.

When it comes to fusing two pieces together, matters get a little more involved. One must ensure that the two pieces are compatible: will fuse together without building in stress when they cool. The simplest way to do this is to cut the two pieces from the same sheet or piece. Unfortunately, this doesn’t give the colour contrast possible by fusing together pieces from different sheets.

These individual sheets may each bear a label certifying them as being ‘tested as compatible’ from one of the reputable glass makers. Provided they are both sourced from the same maker and bear identical labels, one can be reasonably confident that they will be compatible: will fit together.

On the other hand the two pieces of glass can be unmarked, giving no indication of COE, or even of origin. In this situation we must be much more cautious in our approach. Some form of compatability test is needed to give a reasonable guarantee of success. See paper on Compatibility for more on this.

Sheet glass is the most common form of glass used for kilnforming. As it falls into two distinct categories they will be dealt with separately as:

• Float glass
• Rolled glass

Float glass is far cheaper than most rolled glass. because of the quantities produced to keep pace with market demand.The cheapest way to learn the techniques of kilnforming is to use the cheapest glass available.  For most, this is undoubtedly float glass offcuts from the local glazier.

Achieving compatability

Fusing with ‘compatible’ glass is the first essential for success. How one chooses to achieve this will determine, to a great extent, which glass one uses.

One can pay someone to do the compatability testing and select from one of the glass ‘systems’ available. These offer extensive colour and texture ranges which include COE's of 82, 84, 90, 94 and 96. As explained elsewhere, COE isn’t the only determinant of fusing success, but it is a good indicator. Using glass labelled as having been batch tested by the maker is a further safeguard.

Less costly, but demanding of more compatability testing, is to select from the wide range of coloured rolled glass available. Some of this may be labelled ‘fusing standard’, but that’s no guarantee of the compatibility of one sheet with another bearing that label. One must always TEST to be sure.

In particular, System 96 labels should be read carefully. Sheets bearing the ‘System 96 tested compatible’ label can be fused together with a high level of confidence, whereas those bearing a ‘System 96 Fusing standard’ label should be treated with much more caution. Do what the maker advises in his literature and TEST. Don’t assume that glass bearing the ‘System 96 Fusing standard’ label is tested compatible. If it was it would be labelled and priced accordingly. As it isn't so labelled it may have been tested and failed.

Unsuccessful firings can occur on occasion. One of the many reasons can be that the glass wasn’t as compatible as one believed. When searching for a cause of a problem, don’t discount incompatibility simply because of a label. Sheets have been known to be incorrectly labelled from time to time. 

Sensible studio practices can go a long way in ensuring success when fusing. The first one is testing and the second is labelling off-cuts from every sheet tested so that the off-cuts can be matched with surety. The method used isn’t important—using stick-on coloured dots or labels, indelible pencil or pen— are all fine, as long as one devises a system and sticks to it.

One can choose to use glass by one maker and stick to it exclusively, but that cuts one off from all of the other fascinating colour texture and pattern ranges available: and from a lot of experimentation. Better to accept that at some time other glass will ‘sneak in', and be prepared for it.

Bottle glass

Is also a soda lime glass with COE in the same general range as float. Broken bottles can be used to make glass castings, pate-de-verre, or to make stringer and frit in a ‘flower pot’ kiln.

Being commercial products with colour influenced greatly by the demands of the packager, the colours will generally be shades of green brown or blue. A deep coloured bottle glass looks very insipid when drawn out to a thin stringer or flaky shard. Art glass will generally have a deeper depth of colour than bottle glass.

Small chips or stringer from bottle glass is sometimes used as decoration on float. However, it’s very much a ‘suck it and see’ business, as the float could be from anywhere and the bottle from anywhere else. The COE of the two could be vastly different. ‘Vastly’ here isn’t a large number. A difference of two in COE is enough to put the ‘fit’ of the two pieces in doubt.

Some Australian made coloured bottle glass appears to be reasonably compatible with some Australian made float: more so in thin stringer or in small shards which will fuse out thin. Always do a test firing to make sure. See Compatability for more on this. 

Gas in solution

During the manufacture of glass, fluxes can remain as microscopic gas bubbles in solution within the mass. These bubbles will generally be so small as to be invisible to the naked eye and will have no effect on the light transmitting properties of the glass. When the glass is re-heated the bubbles can agglomerate (join together into clusters) or join together and become one larger bubble: to the extent of becoming visible. The greater the number of firings the more likely it is that bubbles will become visible and the larger they will become: even large enough to erupt from the surface.

Treatments during manufacture can reduce the amount of gas in solution so that the formation of visible bubbles after multiple firings is reduced. Fusible glass is designed for multiple firings, so one would expect it to be treated to drastically reduce bubble formation. Visible bubble formation after multiple firings is more likely to occur with some float glass.

There can be a number of reasons other than gas coming out of solution for bubbles to form during kilnforming. See Bubbles in glass for more.

Coatings on glass

There are a number of surface coating processes available, including:

• Iridised sheet, which is rolled sheet to which a surface layer of material has been applied to create varying reflective hues depending on the relative positions of viewer and the light source. Whilst it is used mainly in cold glass work, the glass itself can be slumped or fused: with varying effect on the coating. If fusing, the coated side should be to the outside as the coating can prevent proper amalgamation of the layers of glass.
• Dichroic, which describes a process for the vacuum deposition of microfine particles of crystals or metals in vapour form on the surface of glass or a film. Multi-step processing can produce vast colour combinations. It is available on glass mainly as part of Bullseye and Spectrum/Uroboros ranges, it is a very expensive product. It has only recently become available as a deposit on a paper backing for application to any substrate.
• Silver stain, which is the technique of applying silver nitrate to glass and firing at about 650°C (1200°F) to create a range of transparent yellow coatings.

Of no use to kilnformers are a range of coatings applied to float glass to impart properties such as self cleaning, UV filtering, heat reflecting for use in architectural and structural applications. See Float glass products for more.