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Scrapers & Fabricating Debris Issues - Revised 1/17/07
  Window cleaners: This article will help you and your customers understand the problems presented by fabricating debris on some heat treated glass, and why you need to have a fabricating debris damage liability waiver signed before you scrape tempered glass.

This update contains several clarifications, including the statement that "The incidence of fabricating debris is greatly minimized when temperers adhere to all recommended maintenance procedures for washers, rollers and other tempering equipment."

Print copies and show it to builders. Help them realize why they should insist on quality tempered glass - that can be scraped - and why they need to sign your waiver.

Builders are encouraged to manage fabricating debris as a manufacturing defect on some of the tempered glass they are getting, and address this issue with their suppliers. We've been urging fabricators to test their own glass for this problem since 2005.
This article may be reprinted and redistributed.

  IWCA Seminar on Fabricating Debris Awareness - 2007

Dan Fields, a highly regarded expert on the subject, presented a seminar called "Fabricating Debris Awareness"  February 2007 at the IWCA Convention.
This seminar was recorded by IWCA - but no longer sold

  IWCA Seminar on Glass Fabrication & Processes - 2005
Click here for Gary Mauer's article about the tour & seminar

The day after a tour of the local Arch Aluminum & Glass tempering facility - during their 2005 convention in Orlando - the IWCA recorded a very interesting discussion about how hard these fabricators worked to produce better quality tempered glass for their customers.
During this discussion, these fabricators agreed that an ideal way to check for fabricating debris was to use a razor during regular QC checks on glass exiting tempering ovens.

Window cleaners were bussed to the Arch plant one day before the IWCA seminar. The editor of Glass Magazine - Nancy M. Davis - wrote an account of the discussion at the IWCA/Arch tour for the April 2005 issue. In that article, Max Perilstein, the Vice President of marketing for Arch, and leader of GANA’s Building Envelope Contractors division said, “We believe we're doing things the right way.” He advised window cleaners to “Investigate local glass fabricators. Ask if they’re doing the things we’re doing here.” That article was removed from the Glass Magazine website after only a few days.

This seminar was recorded by IWCA - but no longer sold

Partnering with the Glass Fabricating Industry
International Glass Review is no longer in publication - links to articles are no longer active.
  This 2002 International Glass Review  article by the leading supplier of furnace rollers points out they offer "free seminars to customers focusing on roller properties, cleaning and maintenance procedures, roller refurbishing and regrinding options and precautions, the effects of the plant environment and fabricating practices on tempered glass."

They also offer laboratory analysis as a free service to their clients, to properly identify "glass surface conditions or markings".

At one point, this article says, "Most often the conditions that appear on glass surfaces during the heat treating process are related to foreign materials that have been mechanically impinged into the glass or that has damage the glass as it passes over the rollers. "

An illustrated article from the 2001 International Glass Review entitled,
The dynamics of ceramic rollers

Operating and maintenance practices to produce quality tempered glass; By Renald D Bartoe, is no longer available online from International Glass Review - but back issues can be ordered.

This article also discusses issues related to tempered glass quality from the perspective of the roller manufacturer, and  advocates good housekeeping and good fabricating practices. At one point, this article says, "Glass dust and debris deposited on the furnace rollers can be picked up by the glass." There are several color photos. 
International Glass Review is a tri-annual review of glass production and manufacturing, published by Contract Communications Limited a subsidiary of Euromoney Institutional Investor PLC, in London, England

 4/98 Temper Temper Article in US Glass Magazine (broken links)
Managing the Problems Inherent in Tempered Glass; By: Regina R. Johnson
  This 1998 US Glass Magazine article says in part that "particles such as glass fines from the cutting edging process and handling smudges that are not washed off the surface of glass before tempering will bake on to the surface, causing blemishes..... described throughout the industry by various names, including "orange peel," "heat prickling," "seeds," "bubbles," or just plain "garbage" that was not washed off prior to the glass entering the tempering furnace.  During normal window cleaning after building construction, the pimples can be scraped off and the dragged across the glass surface, causing scratches"
6/99 Maximizing Ceramic Furnace Roll Performance (find this link on Resources page.)
By Renald D Bartoe, Frederick Caillaud, Dr. John Dodsworth and Jerry Osele
  The authors of this 1999 US Glass Magazine article identify glass fines and other surface deposits as defects. The article underlines the importance of plant cleanliness, roller & furnace cleanliness and maintenance, dust control, and states that "Effective use of the glass washer is critical to every tempering operation.  The glass should be washed just prior to tempering... The glass washer must be operating properly with the detergents, brushes and rinse water at the manufacturer's recommended settings.  Maintenance of the glass washer is equally important to ensure the glass is clean and without residue or debris that will be carried into the furnace... "

Bulletins by GANA - (Glass Association of North America)

Proper Procedures for Cleaning Architectural Glass Products
Glass Association of North America
  This GANA bulletin is often interpreted as a  "ban" on scrapers. GANA clearly does not condone or recommend scrapers, and scraping is not considered a "normal cleaning procedure" by GANA,  This bulletin  claims that "widespread" use of scrapers will "often" damage glass. 
Heat Treated Glass Surfaces are Different
Glass Association of North America
  This GANA bulletin refers to the presence of fabricating debris as a "surface condition"  - agrees that fabricating debris may be present on some tempered glass, and also states that scraping can result in damage, "if any microscopic particles have adhered to the surface".

It does not, however, address the variable presence of fabricating debris. On most tempered glass, one side is fine, and the other side may or may not have fabricating debris. There is no explanation for why that occurs, or what is being done about it.

The truth is that proper maintenance in the tempering plant yields a better tempered glass surface - but this GANA bulletin doesn't mention that.

There is also the suggestion that ASTM standards allow the presence of fabricating debris, which suggests that builders have no choice but to accept tempered glass. However, the ASTM C 1036 standard contains disclaimers which seem to contradict that - and clearly, the ASTM standard does not actually state that detectable fabricating debris is acceptable.

The conclusion suggests that "millions upon millions of square feet of glass have been installed with trouble free performance", the inference being that this is because this glass has been cleaned and maintained by professional window cleaners who didn't use scrapers.

A window cleaner's view would probably be that most of those "millions upon millions of square feet of glass" have in fact felt a scraper - with no ill effect, because there was no fabricating debris problem. Scraper use among window cleaners is widespread - nearly universal. Window cleaning scrapers are offered by every major squeegee manufacturer, and sold by every major window cleaning supply house.


6 second audio clip - recorded by Gary Mauer  

You can often detect fabricating debris on clean tempered glass by running a scraper lightly over the surface. However, hearing this sound does not prove the glass will be scratched. (Some fabricating debris won't budge.)

Listen to MP3 file      
Listen to .WAV file (much larger file - slightly better quality)

A microphone was attached to a scraper..... the scraper was placed on a piece of tempered glass, pushed a short way, then lifted and placed on a nearby piece of tempered glass which was loaded with fabricating debris and pushed again - lightly, so as not to scratch the glass. 

The smooth surface that you hear first would have been the top side when that particular piece of glass was tempered, and the rough surface you hear next was the bottom - the side that was touching rollers when that particular piece of glass passed through the tempering oven.

The tinkling sound you hear is little glass fines and other fabricating debris. This is the "tinkle test"....   If you've never heard this before, you'll be impressed - or maybe you'll think we faked it.  

Caution -  Fabricating debris is variable, and this is a more extreme example. It won't always sound this bad, and you cannot count on feeling or hearing this surface defect while you are working - particularly on a messy construction site.  It is also important to realize that hearing this sound does not prove the glass will be scratched. (Some fabricating debris won't budge.)


Miscellaneous Links

Mohs Hardness scale at
This and other Mohs hardness scale web pages rank glass and blades at about the same hardness. Unfortunately, most Mohs hardness scale web pages are written by and for gemologists. Those which do mention glass and blades are similar in hardness tend to mention the fact only in passing.