Tin Bloom

Using float glass sometimes produces partial clouding as though devitrification were present. Although float glass is prone to devitrification, not all the cloudy film on the surface is due to devitrification.

Float glass, which these days, is almost all clear smooth glass, gets its name from the process of floating the glass on molten tin. The tin in compression gives an apparent devitrification effect which is called tin bloom.

it is different from devitrification, to which float glass is particularly subject. Devitrification sprays and solutions will not have an effect on this surface defect called tin bloom. 

When the tin layer is stretched, it does not create a tin bloom on the surface.  Therefore, it is important to have a means to detect which is the tin surface.  Always fire the glass with the tin in the same relative location to each other.  I.e., on several layers of glass have all the tin side down or all up, but not mixed. 

This example of a test by Glass Art by Margot shows the tin bloom on the outer portions of the platter where the tin side was up, causing the tin too be compressed and show.  The flatter portion of the piece did not show this tin bloom as there was not the same extent of compression. You can visit the description of the experiment here.


When forming the glass (slumping, draping, kiln carving) make sure the tin sides will be stretched rather than compressed.  Of course, you can take advantage of the tin bloom by controlling the compression of the tin layers.

Float Glass in the Kiln

An important characteristic of float glass is that a very small amount of the tin is embedded into the glass on the side it touched. The tin side is easier to make into a mirror and is softer and easier to scratch than the air side. The characteristic of float glass having a molecular level of tin left on the “tin side” but not the “air side” is important to distinguish. There are short wave UV light sources to help determine this. The tin side gives a whiter glow than the air side. If any forming of the glass is planed after fusing, the tin side needs to be on the side being stretched, as when in compression the tin side will show a “tin bloom” similar to devitrification.

If the tin side is down on both sheets, and it is slumped into a mould there will be no tin bloom because the tin layer is stretched. If the tin side is up on both sheets and it is slumped into a mould there will be tin bloom because the tin layer is compressed. If you have placed the tin sides together, or on both the top and bottom, one of the tin surfaces will be in compression and so will show tin bloom. This is often mistaken for devitrification, and no amount of any devitrification solution will help.

A borax solution can help with the devitrification on float glass in some circumstances. It is not a perfect solution. This is because tin bloom and devitrification are often not distinguished correctly. But a high level of cleanliness and polishing the glass until squeaky clean is the best start.

The heat characteristics of Float glass depend in large part on which company manufactures the glass being used, so the temperature characteristics are given in ranges.

The softening point is around 760C

The annealing point is around 560—540C

The strain point is around 515-495C. The strain point being the temperature below which no further annealing occurs, although the glass can still be thermally shocked below this range.

Due to the robustness of float glass, it can be fired with a quicker initial temperature rise than glasses formulated for kiln forming. The down side is that it devitrifies very easily and very badly. Rarely can you perform more than two firings before the devitrification begins to become troublesome.

All window glass now seems to be referred to as float glass. However, the float glass process was invented in the 1950’s. Prior to that time, window glass was drawn. Float glass can use more iron in its composition, because it does not have to be drawn up out of a molten vat of glass as the drawn glass did and still does. Float glass is formulated to be stiffer at forming temperatures, whereas the drawn glass has to be flexible due to the mechanical stresses it is put under during the drawing. Except for low iron glass, the float glass has a distinct blue green colour when viewed through the edge. Drawn glass has a variation in thickness and is much paler when viewed through the edge. These visual differences can help distinguish the two kinds of glass, but are not foolproof.

More information on the general characteristics of float glass can be found here.