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Insulated Glass Quality Standards

Stain Patter

An optical effect that results from the tempering process appearing as a pattern of dark spots on the glass. The intensity of the issue increases when viewed at steep angles to the glass.

Allowed. This optical issue is characteristic of tempered glass and cannot be completely controlled. It will vary from pane to pane.


An optical effect that appears as a faint, random pattern resembling an oil stain. 

Allowed. This is the result of having exactly matched thickness of glass panes in an IGU (less than 0.0001" difference in thickness).

Newton Rings

This is an optical issue that appears as concentric rings of rainbow colors near the center of an IGU that has its panes touching each other.

There is no industry standard covering this issue. Contact the manufacturer.

Color Uniformity

Readily apparent glass color variation from window to window, or within a window. It can be the result of the glass tint or glass coating.

Allowed within reason. Can be measured with special instrumentation if needed. Consult with manufacturer.


A moisture or chemical deposit between the panes of an IGU.

Not allowed. Indicates seals have failed or outgassing of materials has occurred.

Sightline Infringement

An extension into the daylight opening of an IGU by the sealant, spacer, or area of coating deletion. 

1/8" maximum 

ASTM provides only basic guidelines for Insulated Glass, please contact Midwest Glass for special requests or more information.

IG THICKENSS CAN VARY +/- 1/16 please call if you need to consult on your IG.

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