Marks in UV Glass


SGF, Supreme Grumble Framer
Resource Provider
Founding Member
Nov 5, 1997
No fixed address, Australia.
Full time grey nomad traveling Oz!
Does anyone in North America have trouble with shadowy marks in Guardian UV glass?
I just had a dozen sheets replaced because it was unuseable with so many marks in it. The replacement batch has them too.
They are marks just below the surface of the glass on the non-coated side. Some of the lines are small and in groups, others run part way or all the way along the sheet.
The marks have a metallic sort of sheen at some angles, but they are translucent. Over a white mat they look like a smudged pencil line.
We tryed one case of Guardian 24x36.......

Got one or two panes that were correct from corner to corner.

Went back to TV where you only get those little specks or bubbles.

You would think in this day of lazer checking equipment....

It really frosts my pumpkin when its where the picture won't hide the flaw, and it's Museum glass....

IMHO: Guardian is cheaper, but it's not less expen$ive.
I have told my supplier about it. They replaced the first batch and I've told them about that lot too and are going to take it back.
I will have to go back to using TV. The disadvantage is the freight cost on TV is around $20/sheet and freight on Guardian is about 50cents/sheet.
Didn't I read here on the Grumble that both companies use the same glass? The puzzling thing is that the flaw is imbedded in the glass. It appears not to have anything to do with the coating.
They do use the same base glass……but not necessarily from the same batch or for that matter not always the same factory…..

Like most manufacturers these days glass manufacturers will use spare capacity at one of there factories around the world to produce a line of glass that may be running short…………..

Keep in mind that an any time around the world there could be a few hundred containers (22 ton per container) of base picture framing glass looking for an end producer/customer…… is all about supply chain management and timely delivery…..

When I had my glass supply business the quality of the glass generally changed a bit from batch to batch……….and one time a batch of glass that was OK on arrival …..deteriorated over a period of one month after a month I just could not clean the glass……I never fully got to the bottom of that one …….though I suspect it was some sort of chemical change…..

I suspect that a bad batch of glass is in Australia…….and most likely in the heel of the hunt it is one of those unfortunate things that is really no bodies fault ….I think it could be a bit of Murphy’s Law ..


After the glass leaves the furnace, it moves along rollers through an annealing oven and down a cooling line then to a cutting area. If one of the hundreds of conveying rollers has a bearing failure and stops turning, the glass will slide over it and get marked. Float glass has what is known as a tin side and an air side. The air side is the side that printers print on or gets coated. The tin side would be the side facing the rollers and thus opposite the UV coated side where the marks are observed.

Why they appear below the surface may be an illusion or they may well be. Not sure of how this could be. It could be voids (bubbles) in the glass that get stretched into long strands during manufacture.

The glass is not coated here in Australia. Its imported from USA. Its not the first bad batch here either from this company. Its been happening since it was first imported.

These marks cannot be felt at all by a sharp blade. If the marks were in the surface I would be able to feel them with a blade quite easily.

This episode is not giving me much confidence in this brand of glass. Makes a person wonder whether they are sending their reject glass down here!
Originally posted by osgood:
...These marks cannot be felt at all by a sharp blade. If the marks were in the surface I would be able to feel them with a blade quite easily...
Not necessarily, osgood. You probably could feel a scratch, but that line may be caused by something else that does not alter the thickness of the coating; at least not enough to notice.

My guess is that the flaw is in the coating, and that it may be found in other lites of glass received in a particular shipment to your supplier. Manufacturing quality monitoring is often done by sampling. When a flaw is discovered in a sample, they try to go back in the order of production and find where it started. Maybe they didn't go back far enough, or maybe it was an intermittent problem that pops up only occasionally...they're the worst kind of production problems.

Guardian makes a lot of the base glass for TV and others, but that manufacturing technology is so refined that flaws are rare. On the other hand, the coating technology is still somewhat difficult, unpredictable, and highly proprietary.
The marks are definitely not in the coating, they aren't even close to the side that has the coating. They are really close to the opposite side, embedded just under the surface.
By placing the glass on a piece of white mat, both ways up in turn, it is easy to tell which side the marks are closest to.
Oh. In that case you have defective glass, and the UV filter might be unrelated to the problem. However, I have seen metallic lines similar to your description appear in the coating.