Air side vs. tin side
Every sheet of float glass has two faces that are not quite the same, one grew up floating on molten tin, the other in open air. Most of the time it does not matter. But when you are placing coatings, laminating, or building an IGU, knowing which face is which is the difference between a product that works and one that quietly fails. Here is why the two sides differ and how to tell them apart.
How float glass creates two different surfaces
Nearly all the flat glass in a building is made the same way: the float process. Molten glass pours out of the furnace onto a long bath of molten tin, where it literally floats, spreading out flat and even under its own weight and surface tension before cooling as it rides down the line. Elegant, cheap, and the reason window glass is so flat, but it leaves a fingerprint.
The surface in contact with the molten tin absorbs small amounts of tin ions into the glass matrix, this becomes the tin side (also called the bottom surface). The upper surface, exposed to the atmosphere of nitrogen and hydrogen gas above the tin bath, develops differently, this becomes the air side.
- Free of tin contamination
- Preferred surface for hard coatings (pyrolytic) and soft coatings (sputtered)
- Cleaner chemical bonding surface for PVB interlayers in laminated glass
- When in doubt, most coating specs default to air side
- Contains residual tin oxide in the glass surface layer
- Fluoresces blue-white under UV light, the easiest field identification method
- Slightly different refractive index than air side
- Some coatings require tin side for adhesion (product-specific)
Identifying air side vs. tin side
In fabrication, every lite needs to be correctly oriented before coating, laminating, or assembling. The standard identification method is a UV lamp, the tin side fluoresces a distinct blue-white glow under UV illumination because of the tin oxide content, while the air side shows no fluorescence.
- UV lamp test: Hold a UV lamp (365 nm / longwave) against the glass edge in a darkened area. The tin side glows; the air side does not. This is the primary in-plant identification method.
- Manufacturer marking: Float glass from primary manufacturers typically has the tin side marked at the edge in print that is readable at the cut facility, though this may not persist to secondary processing facilities.
- Float line markings: Some manufacturers etch or print a mark directly on the glass ribbon; check product documentation.
Coating orientation rules
The coating surface designation in an IGU specification uses a numbered surface convention. Surfaces are numbered 1 through 4 (or more for triple-pane) from outside to inside:
| Surface | Location | Typical use |
|---|---|---|
| Surface 1 | Exterior lite, exterior face | Rarely coated in commercial IGUs; exposed to weather |
| Surface 2 | Exterior lite, interior face (gas space) | Most common location for soft-coat Low-E; faces argon gap |
| Surface 3 | Interior lite, exterior face (gas space) | Second Low-E coating in high-performance or triple-pane configurations |
| Surface 4 | Interior lite, interior face | Sometimes used for hard-coat Low-E; faces room |
When a coating is specified on Surface 2 or 3, the fabricator must verify that the glass is oriented with the correct side (air or tin, per the coating manufacturer's specification) facing the gas space before assembly. Incorrect orientation is a defect that cannot be corrected after IGU assembly.