What is Tin or Float Side of Glass?
Modern distortion-free glass is manufactured by floating molten glass on top of a bed of molten tin. This process is done in order to ensure that the glass is perfectly flat. Molten glass naturally pools to a thickness of approximately .25 inch. The manufacturer controls the speed by which the molten glass travels over the tin, thereby changing the thickness. When you increase the speed, the glass becomes thinner. When you decrease the speed, the glass becomes thicker.
Once the initial batch of finely ground glass ingredients are mixed and melted, it is floated upon a pool of molten tin. The molten tin is called the “float or tin bath”. During this float process, a small amount of tin is absorbed by the glass that is in contact with the tin. This side of the glass is known as the “float or tin side.” The opposite side, which is in contact with inert air, is known as the “air or atmosphere side.” The air side may be slightly contaminated by tin, but not to the extent of the float side.
Identification of the tin side or air side of glass is important, as an end user application may require a specific side of the glass. For example, the air side of glass is best suited for testing, as there is a more consistent chemistry and leads to less variability in test results. The air side is also preferred when fusing glass together. After fusing glass in a kiln, painting must be done on the air side since the paint can react strangely to the tin side. Knowing which side of the glass is air side is important when cutting glass, as glass is always scored on the air side because it breaks more cleanly. Lifting glass using suction cups is best on the air side of glass. Using suction cups on the tin side can cause ghost like images of the suction cups when moisture is present.
The tin side of glass is the choice for certain coatings, since a richer color can sometimes be achieved.
Identifying the Tin of Float Side of Glass
Identifying which side of the glass is tin or float side is very easy using a shortwave ultraviolet lamp. Even a low 4-watt lamp will work just fine, as long as you are in a dark room. When testing your glass, it is important that you wear ultraviolet light eye protection. Any reflective surface, such as glass, can easily reflect invisible ultraviolet light rays into your eyes. Shortwave ultraviolet light or UVC can damage cells on the surface of your eyes. Medically this is called conjunctivitis.
In a very dark room, turn your shortwave ultraviolet lamp on, and place a piece of your glass in front of it. If the tin or float side is facing the ultraviolet lamp, you will see a fluorescent bluish haze from the glass. This happens because the tin or float side of the glass is actually fluorescent under UVC. If you are not sure you are seeing a fluorescent bluish haze, then turn the glass around, and expose the other side to the ultraviolet lamp. You should clearly see a difference as you rotate the glass from one side to the other. Below are some pictures illustrating this tin or float side fluorescent response.
Determining the tin or float side of glass is very important when constructing a fluorescent mineral display case. Some people design their display case using a sheet of glass as their front viewing panel. Some people also use fish tanks to make fluorescent mineral displays (although not the best option). If there will be UVC used in your case or fish tank, it is vital that you determine which side is the tin or float side, and place that side on the outside of your case. If the tin or float side is used on the inside, toward the lamp and specimens, the ambient UVC light will cause the glass to fluoresce. This intense haze obstructs one’s view of the fluorescent mineral specimens inside.
UVC light cannot pass through glass, so if the tin or float side is on the outside of the case, the ambient UVC will not be able to cause a fluorescent response from the glass. Midrange ultraviolet light or UVB also cannot pass through glass. However, longwave ultraviolet light or UVA can pass. If you have a display case that has the tin or float side of the glass on the inside, and you can’t change it, you can use a piece of OP-3 acrylic in front of the glass. OP-3 has the ability to block UVA, UVB and UVC light from passing, all while allowing the visible spectrum to pass. For more information on OP-3 acrylic, please click here.