About float glass

What  it is

Float glass is made through a process of floating molten glass on a bed of molten tin. This produces flat sheet glass that is of a uniform thickness and free of ripples. Look around you and see how our modern urban environment depends on float glass. Pilkington patented the float process in the 1950s, and now it is the dominant process used to make almost all commercial window glass worldwide.

Tin deposit

Float glass picks up a layer of tin on one side during manufacture.

When the glass is re-fired in a kilnforming process the tin can produce a cloudy or hazy effect, known as tin bloom. It is caused by the tin oxidising. The likelihood of it developing is minimised by firing the glass with the tin side down, which not only lowers the temperature of the tin but also restricts the supply of oxygen.

If the bloom appears on a special piece it can be removed by shot blasting or acid etching and the glass can be re-fired to flame polish the affected area: although that may put the entire piece at risk.

A simple method for detecting the 'tin' side is to use an ultraviolet lamp, sometimes erroneously called a  Tin Bloom Detector. It does help find the tin side of the glass, but is not necessary to indicate the tin bloom; that will be all too obvious after the firing. The bloom itself doesn't develop until the glass is being fired.

'Mini Tin Scope' is the name of one recently available device,  as was 'Tindicator'. The latter device was made from a small "Panther" brand battery powered torch such as the Model MTF300 sometimes available from camping goods stores. The ordinary fluorescent tube was replaced by a Philips TUV4 UV tube. The plastic tube cover needed modification as it inhibits UV light. One method is to cover the plastic with black tape and to drill a row of holes to allow the light to exit only from the back. 

Such detectors need to be used in a dark room. Hold the glass roughly vertical and the device in a manner so that the light shines slightly downward onto the glass. The tin will 'fluoresce' in the UV beam; more strongly on the tin side. 

Some fortunate people can detect the tin side with the naked eye.

Others use water and tell by the droplet formation. Smaller pieces can be dipped in water and the droplet formation observed as they drain. The water will run more freely on the non tin side and form bolder droplets on the tin side; stick more to the tin. 

Some religiously check every piece of glass to correctly position the tin side, whilst others pay it little attention, accepting any random effects as being characteristic of the material.

The amount of tin picked up by the glass can vary considerably, as does the extent of the 'tin bloom' appearing on the fired glass; pieces cut from the same sheet can show widely varying effects. Some artists seek to feature this 'smoky' effect, so search for sheets with a high tin coating.

If applying paints to float it is best to put it on the air side and not on the tin side, as tin itself and varying amounts of tin can affect the shade or colour of a coating. (air side is the side facing upwards when the sheet is being made)

COE

Float glass can be sourced from almost anywhere in the world and can have a coefficient of expansion (COE) which varies anywhere between about 80 and the mid 90's.

As these can be mixed up at the distributor level, there is no guarantee at all that any two pieces from a glass merchant will be compatible, will 'fit' together: rather the opposite.

In normal production, the COE can vary by as much as plus or minus 1.5 or so. This means that sheet made in a plant at different times can vary by up to 3 points. As the accepted wisdom is that variation of about 1 point is acceptable, there's no guarantee that two sheets from the same plant made at different times will be compatible.

If buying cut pieces from a glass merchant, be sure to specify that they be cut from the same sheet. But, how can one be sure?

Unless 'you' have cut them from the same sheet yourself, always TEST.

That's a good incentive to learn to cut glass.

Over the years I have accumulated quite a collection of float offcuts from various makers, as evidenced by the range of green edge tints displayed. When trying to fire samples to display incompatibility, it was surprising the number of times that pieces selected with different edge colour would fuse together OK.

Different edge colour is supposed to indicate different makers and the possibility of different COE.

Maybe my Kiln God repellent was working particularly well, maybe luck, or a fluke, whatever. Of course, there's no guarantee that the same combination would work OK next time.

Tint

Impurities in the raw materials can influence the final 'colour' of the glass. It will generally be some shade of green and is generic to the output of a particular manufacturing plant. It is determined largely by the amount and composition of iron impurities in the raw materials.

Because it is the natural colour of the glass it is not regarded as being a tint at all. In fact, this 'normal' glass is commonly referred to as 'clear glass'.

Special additives can clarify the melt so that a sheet of optimum clarity is produced. These are marketed under names such as 'Optiwhite' (Pilks) 'SuperClear' (Viridian) and 'Starphire' (PPG Corp).

Conversely, metal oxides can be added to the melt to either enhance or change the colour of the glass so that an 'all-the-way-through' colour is created. Viridian Vfloat products SuperGreen, Supergrey and SuperBlue are examples of this.

When applied to float glass, tint is taken to mean the 'body tint', or the colour as seen when viewing it from the edge; but not to pale green which is 'clear'. Confused?. Oh well!

While hardly perceived in normal float, this 'tint' does have an effect on the light transmitting properties of the glass.

Normal float, in common thicknesses, blocks about 10 – 15% of the visible light, SuperClear somewhat less, while Supergrey will block up to 90%.

It should be pointed out that 'white' has different meanings when used with float and rolled glass.

When used in reference to rolled glass, white refers to an opaque milky looking product, whereas with float glass 'clear' and 'white' are often used interchangeably to mean clear: as with Optiwhite which is almost colourless.

For the hobby kilnformer, any 'clear' glass which does not have a coating (see below) will meet their need; but the commercial operator has to be more conscious of the effect of mixing sheets of differing tint in one installation, particularly when it is in a light transmitting situation such as in windows.

Starphire is becoming almost a generic term for optimum clarity glass by whichever maker, so the name may no longer accurately identify the origin of the particular sheet.

This is no problem if slumping but, when fusing, like the 'ordinary' glass from different makers they can be incompatible one with another and be susceptible to devit problems to a greater or lesser degree.

The genuine Starphire from PPG has a COE of about 82 and should be compatible with Easy Fuse Float Fit from Youghiogheny Glass. Tests have shown that not to always be the case, so the supposed 'Starphire' may not be from PPG. Recently it has been reported that Starphire was being made in China. It is possible that it will have a COE different from the American product.

How clear is Clear float

The effect of iron impurities in creating the green tint in float glass can be minimised; but rarely overcome completely. When viewed from the edge, any of the special clear products will have a greenish tint; albeit much less than normal float. Problem is to know what is normal.

The only safe way to verify that a piece of glass is clear is to place it on the flat and side by side with a piece of normal float on a sheet of white paper. The difference between the two will be immediately apparent; with the normal float appearing dull and discoloured when compared to the other.

Gas in solution

Fluxing agents and impurities can remain suspended in float as minute gas bubbles too small to be seen by the naked eye.

When re-fired the bubbles can combine to make larger bubbles which can become visible. When multiple firing they can grow to a size where they can erupt from the surface.

Normal float is not made with the intention that it be re-fired, so little attention is paid to this by the manufacturer. However, some makers do offer re-processed products so they are more likely to produce sheet with low gas-in-solution levels.

As float rarely has a makers label, how is one to tell which is which.  TEST.

See 57. Bubbles in glass for more.

Coatings

To meet architectural demand, surface coatings have been developed to reduce the heat flow through the glass and to minimise the need for window cleaning in high rise buildings. Undoubtedly other applications will be introduced over time. Initially applied as soft coatings post production, the coatings are now being applied as the glass is being made and when it is still extremely hot. Thus, they will be inorganic and will be applied to the air side of the sheet, or the side opposite to the tin side.

As more becomes known of the various products, ways to identify them and information on their suitability or otherwise for fusing or slumping will be given in  Float glass products  under coated products.

It should be stressed that coated glass is more expensive and will generally not be deliberately supplied against an order for plain float. However, one cannot discount accident, error, laziness, or other cause. Be aware

Firing float glass

As mentioned elsewhere, float glass is 'tough stuff'. Unlike the makers of fusing glass, the makers of float don't make any recommendations as to firing patterns.

The following results of tests on 2mm, 5mm and 10mm float give some idea of heating rates it can tolerate.

Please note that these heating rates are not given as recommendations and no responsibility is accepted for failure through applying them in another kiln in different conditions. I also acknowledge that I wasn't being very kind to my kiln.

Tests were done in a 25Amp Riley GS-2 kiln using a lightweight calcium silicate shelf to keep the thermal mass low and get maximum rate of heating.

There was no visible change in glass after tests. Truly, float is tough stuff.

The 10mm piece was successfully slumped at 780°C

Sadly, float can also be a bit unreliable. Recently a kilnformed product had to be discontinued because of wide variation in the amount of shrinkage of 6mm thick circles. This occurred after taking all the proper precautions: sourcing from the same supplier, using the same firing pattern in the same kiln, etc.

Although of marginal interest to kilnformers, there is general information on float on websites www.viridianglass.com and www.gjames.com.au

Please note: Pilkington operations in Australia have been bought by CSR Building Products Limited and products made in Australia and previously sold under Pilkington brands will change to Viridian. Extensive changes have been made to Brand names. In particular, Optiwhite is now Vfloat SuperClear. A PDF file downloadable at www.viridianglass.com gives a Product Name Equivalent Chart.

Pilkington continue to manufacture overseas so their brand names and products could continue to appear here if imported by other merchants.