Float glass products

Commercial product

Float is made in a wide range of thicknesses, ranging from 1mm used for laboratory slides and similar applications, 2.1mm through 3, 4, 5, 6mm for commercial glazing then 8, 10, 12 and up to 25mm: but is most readily available in 3, 4, 5 and 6mm sheet.

2.1mm float is used widely in picture framing and is sometimes erroneously called 'picture glass'.

The true 'picture glass' is 2mm thick and has specially treated surfaces to diffuse light and reduce reflections. It is quite expensive relative to standard float. Some can have coatings to inhibit UV light to reduce damage to paintings.

The revised glazing standard introduced in Australia in 2006 has eliminated 3mm float from general glazing use and this will make it less readily available through commercial glass outlets.  

2.1mm should continue to be available through picture framers and some art glass suppliers but will be expensive relative to other thicknesses. If using significant quantities of 2.0mm or 2.1mm, check thoroughly on price as they can vary enormously. The cheapest product is best for kilnforming as the benefits of surface coatings or treatments will all be negated by further processing.   

Thicker glass is sometimes available from broken shop windows. If very old it can be polished plate, rather than float. This is rolled sheet ground and polished to remove ripples.

More recently, shop and commercial applications call for some form of toughened or safety glass.

Safety glass

This exists in two forms;

• Toughened glass
• Laminated safety glass.
  

 Toughened & heat strengthened glass

These are two similar processes which impart different levels of additional strength to float glass.  Toughened glass meets the requirements of Australian Building Codes, is stronger than heat toughened and both are stronger than ordinary float.

They involve the use of specialised commercial kilns and the processes themselves are beyond the scope of this website. However, it is important that kilnformers are aware of these processes which can render the processed glass unsuitable for salvage or rework. Other than for some small identifying stamp, toughened or heat strengthened glass looks no different to other glass

Toughening introduces stress into glass, so trying to cut toughened glass can be quite hazardous as it can easily explode: just like a toughened windscreen when struck with a stone.

However, provided one has a kiln with a hearth large enough to take the full toughened sheet, there are a number of possible options for salvage:

1. The sheet can be fired to about 600°C (1110°F) to relieve the stress. Provided the temperature is not taken too high, and a full anneal process is followed, the sheet can be removed and cut as normal; whilst retaining good clarity.

2. Alternatively, the firing can be taken to a higher temperature and the sheet subject to a full slump.Uneven heating, or too fast a rate of heating, could cause the glass to shatter during the early stages of the above processes.

3. The sheet can be placed on the hearth and then struck with a hammer to shatter it. The broken pieces can then be fused back together, creating unusual patterns of fused cracks.

Some words of caution if following option 3:

• Cover the glass with cloth or heavy plastic when cracking. Of course, one will be wearing safety spectacles whenever carrying out such operations.
• Do this operation only on a brick hearth. If doing it on a kiln shelf or slab, the blow which cracked the glass could also break the shelf.

The broken pieces can push apart as the energy is released, and by varying amounts. Remember the equilibrium thickness. 7mm. During firing, 6mm glass will roll back and shrink in size, so some of the cracks may not rejoin. 8mm or 10mm glass will spread outwards and some cracks could disappear.

Whilst they do work, no guarantee is given as to the success of the procedures outlined above.

Laminated safety glass.

Safety glass is being increasingly supplied in laminated form, where two sheets of float are bonded together using a plastic intermediate layer.

Whilst the plastic bonding layer can be burnt out, such a procedure is not recommended, for a number of reasons including:

• Potential health hazard from fumes and emissions.
• Likelihood of the glass breaking due to uneven heating as the plastic burns in from the edges during the firing. 
  

Special float products

Float is being given treatments to enhance existing or to impart new properties; including enhanced resistance to the transmission of UV light or a coating to make the glass self cleaning.

Whilst the properties themselves are of little interest to kilnformers, people using commercial glaziers' offcuts  should be aware of the possibility of receiving such products.

Treatments can be either additives incorporated into the melt or coatings applied during float production.

The nature of such treatments are not known, so comment at to the safety or otherwise of firing them cannot be made. Be aware that they exist. The rule applicable to food may be appropriate here; if in doubt, throw it out.

Coated products

On line, or pyrolitic coated products.

These are coatings applied as gas vapours when the glass is at a high temperature on the float line; so will be on the air side or opposite the tin side. Coatings are hard and durable and can be applied to any float product, be it normal green tint, low iron or full body tinted.

Coatings are metallic so are electrically conductive to some degree and can be detected using a normal multimeter on X1 or X10 settings, with considerable variation between coatings with different functions. The resistance generally increases considerably after being re-fired to 800 deg C.

Coatings generally don't like being slumped as into a mould as they don't stretch or shrink with the underlying glass, instead showing wrinkles or stretch marks; or may become bubbled or discoloured. Light transmittance may change after re-firing.

Fusing coated side to coated or to tin both bond successfully but in neither case does the edge develop as would similar layers of plain glass.

Overall, coated glass is not suitable for slumping or fusing.