Just saw This Old House with a factory tour of glass-making. The process of making float glass has always intrigued me because I can't figure out how floating molten glass on a bath of molten metal can result in such a perfectly flat surface. Aren't there ripples to worry about? And how is this better than a solid polished surface to receive the molten glass? Is there further cold polishing required to get the final surface?
If you ever saw a pool of mercury or molten tin or indium, you would see just how perfect a mirror the surface creates. Since it is rare for a person to see these things (unless they are a metallurgist), the high optical perfection of such a pool is not appreciated. Glass is viscous at those temperatures and so will not ripple from vibrations. How smooth is the surface of a pool of oil on the floor? The surface of liquid is better than that from 'cold' polishing any day.
Dr. K
It's surface tension that makes the glass surface flat. It needs no polishing after manufacture. The natural thickness is 6mm, which is what you get if you float a body of molten glass on the tin. To go thicker the edges have to be constrained so that it cannot spread fully, and to go thinner, the edges have to be held while the ribbon is stretched mechanically, so that it does not neck down to a narrower ribbon.
The advantage lies in the capital and operational cost, which is far lower than the twin grinding lines which the float process replaced. Those had very complex mechanical grinding systems, both above and below the glass ribbon to grind both sides simultaneously. Later the float process became sufficiently economical to replace the sheet glass drawing processes.
So what about the other side? How does it get so flat?
That's the result of the interfacial energy between the glass and the tin. The density difference is such that there is very little glass below the normal tin surface.
As you can imagine, the surface properties for the "float side" and "non float side" are different. This can influence the performance of adhesives and coatings.
Float side will often glow on exposure to black light (UV)
RT
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