Industry Glass Standards

Click Here for PDF Version

Before pointing out defects and imperfections, understanding ASTM inspection criteria is very important. It outlines things such as inspection distance, lighting requirements, inspection times, etc.

A certain element of visual distortions in glass are accepted in glass manufacturing. The glass that is ‘processed’, such as tempered glass, goes through additional manufacturing processes which can change the visual appearance of the glazing.

Damaged glass is always classed as unacceptable and will not be installed on-site, however, there are a few types of visual distortions in glass that are well documented and accepted as a natural element of processed glass.

Anisotropy (or Strain Pattern)

Anisotropy is a specific visual effect that can be seen in some tempered glass panels under polarized light. It is caused by the different layers of ‘stress’ or tension within the thickness of a tempered glass unit. When polarized light passes through these areas of the glass it can appear as an iridescent or colored pattern on the surface of the glass.

As polarized light is present within normal daylight, Anisotropy may be visually apparent at certain times of day or under certain lighting conditions. This type of visual distortion in glass is often more apparent when the glass is viewed obliquely or when the glass is installed at right angles to each other. This visual phenomenon can also be known as a Strain Pattern, Quench Marks, or Leopard Spots. Anisotropy is an accepted characteristic of the glass tempering process and is not classed as a glass defect.

Roller Waves

Roller Waves is a common type of visual distortion in glass that may be visible in

strengthened glass units. It is caused during the tempering process when the heated

and slightly molten glass is passed over large rollers whilst it is cooled. As a result,

the glass may have a slightly waved finish to the surface. This does not affect the

strength or durability of the toughened glass and is only visually detectable under

certain circumstances.

As a rule, the visual appearance of Roller Waves in glazing is not classed as a glass

defect. However, at Blueline Glass we follow the ASTM Guidelines to determine

what is an acceptable level of roller wave and what is not.

Newtons Rings (Interference Fringes)

Newton Rings are the most common example of Interference Fringes in insulating glass as they are often the easiest to see. They are caused when two panes of an insulated glass unit touch each other.

Where the glass units connect you will see a series of concentric fringes that surround the point of contact.

The visual distortions in glass are caused by the delay in light reflection between these two surfaces of glass causing two light waves to combine.

The presence of a Newton Ring in an insulated glass unit may be a result of a narrow spacer bar being used within the insulated glass unit. If the spacer bar used in your IGU is too narrow the natural deflection in the glass units may cause the two panes to touch.

In some instances, changes in air pressure (ie if the glass is being installed at a high altitude) between where the glass unit was sealed and where it is installed may cause the glass to touch at the center.

It is always highly important for you to let your architectural glazier know exactly how your glass installations will be used and where they are located. This will enable them to properly specify the best glass makeup.

Brewster’s Fringes (Interference Fringes)

Brewster’s Fringes are another form of Interference Fringe found in

architectural glazing installations. These visual distortions in glass are less

common as they are rarely noticeable under normal lighting conditions.

These interference fringes are actually caused by the use of high-quality

glass with optically flat surfaces that are used parallel to each other

(ie in an insulated glass unit). Where both panes of high-quality glass within

the unit are the same thickness, light reflected in one glass can combine

with that from the other. As the light path differences are so small (due to

the high quality and precision manufacturing of the glass panes) these can

cause interference.

The visual distortions in glass and the effect seen is a series of faint bands or

irregular shapes (normally grey or very faintly colored) which could be seen in any area of the glass unit. However, it is very difficult to see a Brewster’s Fringe under normal conditions. In most instances, you will only be able to notice these interference fringes under special

lighting or viewing conditions (such as viewing the glass at an angle with a shaded area beyond the glass).

You can check if your visual glass effect is a Brewster’s Fringe by lightly tapping or pressing the glass. If it is a Brewster’s Fringe the fringes will move in response to the small changes in the geometry of the parallel glass panes.

These visual distortions in glass are not manufacturing faults or defects and glass displaying this fringe pattern does not need to be replaced.

Inspecting Visual Distortions in Glass

All glass is visually checked when we receive it from our fabricator and then again when installed on site. If any glass unit does not adhere to our strict quality guidelines it will be rejected and replaced.

We inspect glass using the guidelines from the ASTM which is the government-recognized trade body for the glazing industry. All glass units to be inspected must be done under the strict guidelines set out to determine whether a glass unit is ‘acceptable’ or ‘unacceptable’. These guidelines include rules for how we have to inspect the glass:

You should stand no closer than 8' -10' away from the glass face.

You should face the glass head-on at a 90-degree angle.

Glass inspections should be done in natural daylight but not directly towards the sun and with no visible moisture on the glass.

You should look through the glass rather than at the glass.

Other Visual Distortions in Glass

If there are other visual distortions in glazing they should be inspected to

determine what they are and if they are classed as acceptable within the

guidelines from the ASTM.

To inspect the glass we will use the guidelines detailed above whilst looking

through the glass in natural light. The glass will be classed as ‘acceptable’ if the

following are neither obtrusive nor bunched together:

Bubbles or blisters

Fine scratches, no longer than 25mm

Minute particles

In addition, Metal Oxide coatings such as Low E Coatings may produce

momentary visual effects on the surface of the glass.

Suction Marks on Glass

When glass is being manufactured and installed rubber suction cups are used to move glass without damaging it. Sometimes, these suction cups can leave an invisible residue on the glass surface that dissipates over time.

The presence of this material on the surface of the glass can sometimes be visible when moisture settles on the surface (such as when condensation forms on the glass).

This is nothing to worry about and is not a defect in the glass. The rubber particles will wear away over time under normal exposure to the elements. They can sometimes be minimized or removed with the use of glass cleaning materials.

EXAMPLES OF VISUAL INSPECTION CRITERIA

Glass shall be inspected in the central (see illustration and categories below) area in transmission at a distance of 5 feet from the observer, and in reflection at a distance of 8-10 feet from the observer.
Glass shall be inspected in the border area in transmission and reflection at a distance of 8-10 ft from the observer.
The glass must be inspected using a viewing angle of 90 degrees from the glass with a suitable background light (daylight without direct sunlight or a range of 500 – 1000 foot-lamberts.) If a lighting box is used as a light source, the diffusing plate shall be parallel to and at a distance of 10 ft from the glass.
Inspection should not exceed viewing of more than 5 seconds for lites up to 6 square feet, 10 seconds for lites up to 35 sq ft, and 20 seconds for lites larger than 35 square feet, either in transmission or reflection.
If defects are visible beyond what is allowable as listed by sizes (square feet) below using the inspection criteria, the glass may be rejected.

ASTM STANDARDS FOR GLASS: ALLOWABLE DEFECTS

a) Single (individual) lites up to 6 square feet.

Scratches – Viewable as stated above must be 1” or less. No more than 1 is allowed per lite.
Debris, Dirt, Spots – Viewable as stated above, must be 1/16” or less. No more than 1 is allowed per lite.
Seeds, Bubbles, Knots, Stones – Viewable as stated above, must be 1/16” or less. No more than 1 is allowed per lite.
No more than 1 total viewable defect as described above is allowed per lite.
Shells – No more than ¼” from the edge less than ½ the thickness of the lite. No more than 1 per side.
Chips – No more than 1/8” from the edge. No more than 1 per side. (see illustration for chips.)
Coating – Must be uniform on the lite inspected when viewed in transmission using the inspection criteria previously stated above.
The border area is comprised of 2 inches plus ½” from each edge of the lite. All other area is the central area (see illustration.)

b) Single (individual) lites 6 to 35 square feet.

Scratches – Viewable as stated above must be 1” or less. No more than 2 are allowed per lite and must be separated by a minimum of 24 inches.
Debris, Dirt, Spots – Viewable as stated above, must be 1/16” or less. No more than 2 are allowed per lite and must be separated by a minimum of 24 inches.
Seeds, Bubbles, Knots, Stones – Viewable as stated above, must be 1/16” or less. No more than 2 are allowed per lite and must be separated by a minimum of 24 inches.
No more than 2 total viewable defects as described above are allowed per lite.
Shells – No more than ¼” from the edge less than ½ the thickness of the lite. No more than 1 per side.
Chips – No more than 1/8” from the edge. No more than 1 per side. (see illustration for chips.)
Coating – Must be uniform on the lite inspected when viewed in transmission using the inspection criteria previously stated above.
The border area is comprised of 4 inches plus ½” from each edge of the lite. All other area is the central area (see illustration.)

c) Single (individual) lites over 35 square feet.

Scratches – Viewable as stated above, must be 1” or less. No more than 3 are allowed per lite and must be separated by a minimum of 24 inches.
Debris, Dirt, Spots – Viewable as stated above, must be 1/16” or less. No more than 3 are allowed per lite and must be separated by a minimum of 24 inches.
Seeds, Bubbles, Knots, Stones – Viewable as stated above, must be 1/16” or less. No more than 3 are allowed per lite and must be separated by a minimum of 24 inches.
No more than 3 total viewable defects as described above are allowed per lite.
Shells – No more than ¼” from the edge less than ½ the thickness of the lite. No more than 1 per side.
Chips – No more than 1/8” from the edge. No more than 1 per side. (see illustration for chips.)
Coating – Must be uniform on the lite inspected when viewed in transmission using the inspection criteria previously stated above.
The border area is comprised of 6 inches plus ½” from each edge of the lite. All other area is the central area

Glass Inspection for the Homeowner

The glass industry has developed consensus standards/specifications that address quality aspects of various types of glass used in windows, doors, and skylights. These standards are very comprehensive and typically written for manufacturers who have expertise in applying them. The standards are also very specific to a given glass type. These types include raw flat (annealed) glass, heat-treated glass (ex. tempered), coated glass (ex. low-E), laminated glass, and IGUs. Unfortunately, the methods of inspection and the blemish or issue criteria are not always the same for each type of glass.

In most cases, a homeowner will have a window, door, or skylight that contains an IGU that is comprised of two or more panes of flat glass. Typically, this glass will also have some type of low-E coating. If there is a safety concern or a code requirement for safety glazing, the window may also contain tempered or laminated glass. With a combination of all these glass types in one window, door, or skylight, sorting out the appropriate criteria to the respective standard is at best a complicated, if not confusing, endeavor. How then is a homeowner to know whether an issue or blemish is a legitimate concern?

The following is a general guideline offered to simplify the inspection of glass issues for the homeowner. This is not necessarily a definitive guide covering all glass-related concerns; that’s the job of the industry standards. Rather, this guideline is a check for a homeowner to flag whether an issue merits consideration for replacement.