Float Glass Characteristics

[Kev]

Float Glass: How does it compare to the System 96 schedule for fusing, using glass of a comparable thickness? Any special considerations to be aware of in using float glass?

[Stephen Richard]

Having spent some time preparing an answer to a question, only to find the place to which it should be posted has disappeared, I am posting it here in the hopes it will be picked up and transfered to the appropriate place.
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"What are the characteristics of float glass and how do they relate to S96?"

A reported 90% of the world's flat glass is produced by the float glass process invented in the 1950's by Sir Alastair Pilkington of Pilkington Glass. Molten glass is “floated” onto one end of a molten tin bath. The glass is supported by the tin, and levels out as it spreads along the bath, giving a smooth face to both sides. The glass cools as it travels over the molten tin and leaves the tin bath in a continuous ribbon. The glass is then annealed by cooling in a lehr. The finished product has near-perfect parallel surfaces.
An important characteristic of the 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.

Float glass is produced in standard metric thicknesses of 2, 3, 4, 5, 6, 8, 10, 12, 15, 19 and 22 mm. Molten glass floating on tin in a nitrogen/hydrogen atmosphere will spread out to a thickness of about 6 mm and stop due to surface tension. Thinner glass is made by stretching the glass while it floats on the tin and cools. Similarly, thicker glass is pushed back and not permitted to expand as it cools on the tin.


The 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—550C
The strain point is around 525-505C.  The strain point being the temperature below which no further annealing can occur, but the glass can still be thermally shocked below this range.

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.  If any forming of the glass is planed after fusing, the tin side in compression will show a “tin bloom” similar to devitrification.

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.

The question asks about comparison to S96.
The softening point is also in a range 674 – 686C
The annealing point is between 507 and 519C
The strain point is between 470 and 482C
[these are according to the S96 website]


The temperatures for Bullseye Glass are similar for each point.

The temperatures for Artista are similar to Float glass, but it is not such a difficult glass to work with.

[Kev]

"If any forming of the glass is planed after fusing, the tin side in compression will show a “tin bloom” similar to devitrification."

Compression as in being slumped into a mold tin side down? Or did you mean to say "in comparison"

Interesting info..thanks!

[Rebecca]

Does the borax solution help the devit for float glass?

Rebecca

[Judy K]

When I have fused float, I fuse it to itself from the same sheet of glass. That way I know it is completely compatible. Since I am usually wanting an ice immitaion , snow flakes ect. , I don't mind some divit. I have used the Super Spray from Fusemaster and it does help but does not solve the divit problem.

I found different floats have a different color. The modern stuff is blue green when 3 layers thick. The pre 1950's is yellow or clear/white.

[Kev]

Interesting Judy!

[Stephen Richard]

"If any forming of the glass is planed after fusing, the tin side in compression will show a “tin bloom” similar to devitrification."

Compression as in being slumped into a mold tin side down? Or did you mean to say "in comparison"

Interesting info..thanks!

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 devit solution will help.   

[Stephen Richard]

"Does the borax solution help the devit for float glass?"

Yes, as Judy says.  Also as she says 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 squeeky clean is the best start.

[Stephen Richard]

Judy K says  "I found different floats have a different color. The modern stuff is blue green when 3 layers thick. The pre 1950's is yellow or clear/white. "

This is because, as indicated at the top of the long post, it is not float glass.  Float glass 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.

[Judy K]

 

I have access to some of that old glass and will treat it with more respect from now on.

[Kev]

Thanks for that explanation Stephen, it made perfect sense to me and I now understand the implications of the tin side vs the non tin side.