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Tempered Glass


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I used to pick up glass at a tempering plant in Chicago for a glass company. They hung the glass by tongs not too different than the tongs ice men would use to carry a block of ice. The tongs combined with the heat would leave tiny dimples in the glass. The dimples would be along one side of the glass and are pretty obvious. This was a vertical furnace. The horizontal furnace would leave roller-wave distortion in the glass and operators would use a "zebra board" to check if product was in tolerance. From what they told me, there is always some distortion in the glass. The board is held up to the glass and the operator would examine the reflection for distortion.

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Does anybody know a way or instrument to test for tempered glass (besides a ball peen hammer)?

I am getting questioned on recommendations when there is no marking on the glass (how do you know it isn't tempered?).

Yes. Go to an old camera shop (if there still are any in your area) that deals in used equipment and buy yourself two polarizing filters.

Hold one filter on one side of the glass and the other filter on the other side of the glass. Align your eyeball so that it's looking through both filters, like a telescope. Then rotate one of the filters till the image that you see through them darkens. Now move the filters back & forth over the glass. If it's tempered, you'll be able to clearly see the stress lines in the glass.

- Jim Katen, Oregon

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Identifying the type of glass if not marked is one thing. Another is the requirement for it to be marked. So, if the location is such that tempered glass should be used, it's supposed to be marked. If it's not marked, it's wrong on that account in addition to whatever else.

From the '06 IRC:

R308.1 Identification.

Except as indicated in Section R308.1.1 each pane of glazing installed in hazardous locations as defined in Section R308.4 shall be provided with a manufacturer’s designation specifying who applied the designation, designating the type of glass and the safety glazing standard with which it complies, which is visible in the final installation. The designation shall be acid etched, sandblasted, ceramic-fired, laser etched, embossed, or be of a type which once applied cannot be removed without being destroyed. A label shall be permitted in lieu of the manufacturer’s designation.

Exceptions:

1. For other than tempered glass, manufacturer’s designations are not required provided the building official approves the use of a certificate, affidavit or other evidence confirming compliance with this code.

2. Tempered spandrel glass is permitted to be identified by the manufacturer with a removable paper designation.

R308.4 Hazardous locations.

The following shall be considered specific hazardous locations for the purposes of glazing:

1. Glazing in swinging doors except jalousies.

2. Glazing in fixed and sliding panels of sliding door assemblies and panels in sliding and bifold closet door assemblies.

3. Glazing in storm doors.

4. Glazing in all unframed swinging doors.

5. Glazing in doors and enclosures for hot tubs, whirlpools, saunas, steam rooms, bathtubs and showers. Glazing in any part of a building wall enclosing these compartments where the bottom exposed edge of the glazing is less than 60 inches (1524 mm) measured vertically above any standing or walking surface.

6. Glazing, in an individual fixed or operable panel adjacent to a door where the nearest vertical edge is within a 24-inch (610 mm) arc of the door in a closed position and whose bottom edge is less than 60 inches (1524 mm) above the floor or walking surface.

7. Glazing in an individual fixed or operable panel, other than those locations described in Items 5 and 6 above, that meets all of the following conditions:

7.1. Exposed area of an individual pane larger than 9 square feet (0.836 m2).

7.2. Bottom edge less than 18 inches (457 mm) above the floor.

7.3. Top edge more than 36 inches (914 mm) above the floor.

7.4. One or more walking surfaces within 36 inches (914 mm) horizontally of the glazing.

8. All glazing in railings regardless of an area or height above a walking surface. Included are structural baluster panels and nonstructural infill panels.

9. Glazing in walls and fences enclosing indoor and outdoor swimming pools, hot tubs and spas where the bottom edge of the glazing is less than 60 inches (1524 mm) above a walking surface and within 60 inches (1524 mm) horizontally of the water’s edge. This shall apply to single glazing and all panes in multiple glazing.

10. Glazing adjacent to stairways, landings and ramps within 36 inches (914 mm) horizontally of a walking surface when the exposed surface of the glass is less than 60 inches (1524 mm) above the plane of the adjacent walking surface.

11. Glazing adjacent to stairways within 60 inches (1524 mm) horizontally of the bottom tread of a stairway in any direction when the exposed surface of the glass is less than 60 inches (1524 mm) above the nose of the tread.

Exception: The following products, materials and uses are exempt from the above hazardous locations:

1. Openings in doors through which a 3-inch (76 mm) sphere is unable to pass.

2. Decorative glass in Items 1, 6 or 7.

3. Glazing in Section R308.4, Item 6, when there is an intervening wall or other permanent barrier between the door and the glazing.

4. Glazing in Section R308.4, Item 6, in walls perpendicular to the plane of the door in a closed position, other than the wall toward which the door swings when opened, or where access through the door is to a closet or storage area 3 feet (914 mm) or less in depth. Glazing in these applications shall comply with Section R308.4, Item 7.

5. Glazing in Section R308.4, Items 7 and 10, when a protective bar is installed on the accessible side(s) of the glazing 36 inches ± 2 inches (914 mm ± 51 mm) above the floor. The bar shall be capable of withstanding a horizontal load of 50 pounds per linear foot (730 N/m) without contacting the glass and be a minimum of 1½ inches (38 mm) in height.

6. Outboard panes in insulating glass units and other multiple glazed panels in Section R308.4, Item 7, when the bottom edge of the glass is 25 feet (7620 mm) or more above grade, a roof, walking surfaces, or other horizontal [within 45 degrees (0.79 rad) of horizontal] surface adjacent to the glass exterior.

7. Louvered windows and jalousies complying with the requirements of Section R308.2.

8. Mirrors and other glass panels mounted or hung on a surface that provides a continuous backing support.

9. Safety glazing in Section R308.4, Items 10 and 11, is not required where:

9.1. The side of a stairway, landing or ramp has a guardrail or handrail, including balusters or in-fill panels, complying with the provisions of Sections 1013 and 1607.7 of the International Building Code; and

9.2. The plane of the glass is more than 18 inches (457 mm) from the railing; or

9.3. When a solid wall or panel extends from the plane of the adjacent walking surface to 34 inches (863 mm) to 36 inches (914 mm) above the floor and the construction at the top of that wall or panel is capable of withstanding the same horizontal load as the protective bar.

10. Glass block panels complying with Section R610.

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Hold one filter on one side of the glass and the other filter on the other side of the glass. Align your eyeball so that it's looking through both filters, like a telescope. Then rotate one of the filters till the image that you see through them darkens. Now move the filters back & forth over the glass. If it's tempered, you'll be able to clearly see the stress lines in the glass.

I'm thinking I'll need Marcel Marceau to hold the filter on one side for the fixed pane windows. My arms just aren't that long.

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Hold one filter on one side of the glass and the other filter on the other side of the glass. Align your eyeball so that it's looking through both filters, like a telescope. Then rotate one of the filters till the image that you see through them darkens. Now move the filters back & forth over the glass. If it's tempered, you'll be able to clearly see the stress lines in the glass.

I'm thinking I'll need Marcel Marceau to hold the filter on one side for the fixed pane windows. My arms just aren't that long.

Yes. If your arms won't reach around the glass you're back to looking for tong marks or etchings.

There are some old glass guys in my area who claim that they can tell tempered glass by the sound it makes when they rap on it. I've stood in the glass shop where one of them demonstrated it to me. With two pieces of glass in the shop, right next to each other, I could hear the difference, but when I get to a house, with glass in frames, I can't.

- Jim Katen, Oregon

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Nearly all of the tempered glass I see has clearly visable roller distortion, it can be a pain to spot in small pieces of glass but I can usually see it.

There won't always be markings on tempered glass either. Just last week I installed an 8' wide sliding window and the center sash was tempered to meet manufacturing requirements (the end sash were not tempered), no mention of tempering on the label, packing list or invoice, and no etching on the glass either. The roller distortion was quite obvious though.

The exceptions that John pointed out are a good thing. I was once in a manufacturer showroom that had a very expensive mahogany 15 lite insulated glass french door on display, each individual lite had two 'bugs' on it, one on each side of every lite. Pretty hideous.

Tom

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