Is there a way to determine if glass is tempered?

[Kelly Thorson]

Other than hitting it with a hammer? I thought I had read the answer to this once, but didn't have any luck with a search.
I scored myself a beautiful 1/4 x 5'x3' mirror at a garage sale (for a whopping $15 ). At first inspection it appears to be in pristine condition. The fellow who sold it to me said he gave it to his parents in the 40's. It came clipped to a backing frame which is perfect for the table project I have been wanting to do. I plan to use photo stencil to blast an old map of the world into the centre of it, and will need to get the stencil done professionally so I don't want to take a chance that the glass will shatter (the lines need to be deep enough to facilitate being scraped over with a razor and retain their paint). Is it possible to get the surface tension tested anywhere or is there any other way I can tell?
Another question, will removing the mirroring leave any residue that will interfere with a water gild? The border of the table will be done with a "Verre Eglomise" technique.
Thanks in advance for your input.
Kelly

[Larry White]

Hi Kelly-
The tempering process will distort the perfectly flat surface of standard annealed glass. If you silver a piece of tempered glass and make it into a mirror, you pretty much end up with a funhouse mirror. The mirror you have would not have been created on tempered glass. I have removed the backing of old mirrors with paint stripper, the removed the old silver with silver strip. Typically these mirrors are beveled, I like to leave the old siver bordering the bevel and execute the artwork in the center.

As for knowing if a piece of glass is tempered or not, first look for a tempering logo etched into one corner of the glass. Next would be to inspect any reflection in the glass surface for distortion, this would indicate it's probably tempered. If you're not sure, just treat it like it was tempered. About the only thing you can't do with tempered glass is depth carve it. All the large signs in my shop are executed on "scrapped" tempered glass.



Here's one I did for a friend...

Proceed.....

[Kelly Thorson]

Thanks Larry
I found a method where you can use polarizing filters to determine if glass is tempered but I really didn't want to strip the mirroring off if it wasn't going to work. There is no distortion in the mirror so I will assume it is not tempered and strip it. I'm looking forward to getting started on this project, maybe this find will give me the jump start I needed.
Nice piece for that friend of yours, I knew he was old but wow..... . Hope you had a great weekend!
Kelly

[Raymond Chapman]

One method is to hit it with a hammer. If it breaks into millions of little pieces it's tempered. If if breaks into large shards, it's not tempered.

Hope this helps.

[Kelly Thorson]

Thanks Ray, boy it must be tough getting old, by the time you read through the post you forgot I started my post with "Other than hitting it with a hammer. "
Must be past your nap time. Thanks for the smile.

[Raymond Chapman]

Oops. Sorry Linda.

[Billy Pickett]

...Kelly, Rick G. sold something which I think he called a "mineral light". Which was like a small BLACK (or purple) light that could disclose which side was which on tempered glass. I think he believed (or knew) that one side was (more) preferred for gilding on, than the other. ...Perhaps one of these is what you need.

[Raymond Chapman]

I think those lights were just for determining which side of the glass was the "tin" side and not if it were tempered or not. But, I may be wrong. I think it's happened before.

[Kent Smith]

Ray is right. More critcal with silvering and especially angel gilding.

[Dan Seese]

I could've used some of this advice a few months back.

About 10 years ago I did a window for a business here in Fort Collins, "Cache La Poudre Rifleworks".



The owner died a couple of years back and his brother finally decided to close the store. I thought I might salvage glass, cut out the portion of the glass that had the incription on it, frame it and hang it in my shop.

Though the windows were quite large, there was no tempered stamp at the bottom. A glass shop looked at it and determined that it must be annealed plate glass so I paid them to remove it and replace it.

It was an insulated unit so when they tried a test cut on the clear glass pane, they couldn't get the break to follow where they had scored the glass. "Must be heat-strengthened glass" they told me - which is sort of a step between regular plate glass & tempered glass.

I took a scrap to a couple of water-jet places & one of them were able to cut out a nice little square. When we took the large window there, using all kinds of precautions I was hopeful. As soon as the water jet stream hit the glass I heard a loud "pop". Unlike tempered glass, which would have resulted in thousands of nice little nuggets, this ended up in about 20 pieces. I kept it for a while but then finally hauled it to the landfill a couple of months ago.

Wickipedia (that inerrant authority on all things) has this to say: "Heat-strengthened glass is glass that has been heat treated to induce surface compression, but not to the extent of causing it to "dice" on breaking in the manner of tempered glass. On breaking, heat-strengthened glass breaks into sharp pieces that are typically somewhat smaller than those found on breaking annealed glass, and is intermediate in strength between annealed and toughened glasses."

I still don't know if there is a way to tell that glass is "heat strengthened" but it would have saved me about $1000 if I had known.

Live and Learn!

[Kelly Thorson]

That's beautiful Dan. I can see why you would have wanted to keep it.
I'd love to see more of your work.
Heat strengthened glass - a whole new angle?

[Jerry Berg]

So, what should we be looking for in finding the appropriate glasss for a job? Best for gilding, best for chipping and best for sand carving, and why? Not trying to take this off topic Kelly, I gots ta know!

BTW Dan, that is some very beautiful work. Sorry you lost it. Is the painted gun and powder horn brushed or airbrushed.

[Patrick Mackle]

Besides looking for heat flairs, bows, warps or micro impressions left in the surface from the conveyor rollers in the tempering oven I use polarized lenses to look for stresses in kiln molded and tempered glass. Glass that has been altered from its factory annealed state will show up as rainbow patterns when viewed between polarized lenses.
I'm not sure if they will detect chemically strengthened glass. The glass is best viewed at the corner, so if the glass is in a setting with wooden stops or in a channel it may be difficult to view a corner, unless your lenses are cut square so you can get very near a corner.
To check the glass for stress or tempering, situate the glass between the two lenses against a bright light source. Then slowly turn only one lens. In the turning, as the lenses darken, you will see the stress area in the glass begin to appear as a bright "rainbow" type flair. Glass that exhibits this property will explode or run a break if worked on in a way that will promote/release that stress.
The polarized lenses I use are about 5" X 7" X 1/16" thick. I think they are actually polarized LEXAN plastic. Very handy around the shop for forming glass in kilns and detecting customer supplied tempered glass before attempting to cut or sandblast.
Pat

[Patrick Mackle]

So, what should we be looking for in finding the appropriate glasss for a job? Best for gilding, best for chipping and best for sand carving, and why? Not trying to take this off topic Kelly, I gots ta know!

BTW Dan, that is some very beautiful work. Sorry you lost it. Is the painted gun and powder horn brushed or airbrushed.

I have done some work on plate glass made in France by Saint-Gobain.
It is nice glass, with a lighter yellowish green cast when viewed from the edge than american iron plate glass, not unlike the old softer "boiler plate" glass found in old store front windows of the early-mid 1900's.
It looks similar in color to some old 1" thick seafoam green glass tops in an older bank here in town- beautiful thick glass!! It would be nice to know how it does when glue chipped.
Pat

[Danny Baronian]

What glass to use for a job?

Depends on the location, use and city requirements. At the very least in a public place, glass 18" or lower should be tempered and have a 'bug' stamped in the corner indicating that it has been tempered. In my city, the final inspection will not be signed off unless bugs are clearly visible.

It's possible to use laminated glass which in some cases can be used in place of tempered. In the event it needs to be cut, laminated glass can be cut, chipped and depth carved to half it's thickness. If it's for a city permited job, call the inspector. I did a job locally to the customers specifications, plate glass, depth carved and acid etched.

I was paid to duplicate the same job with a second set in tempered glass before the city would sign off.

As far as the mineral light for gilding it's not needed. It was was a step Rick used to eliminate any possible cause for angel gilding failure with his forumla. The kit's sold now are more consistent and reliable as the forumla is different.

Dave Smith's been angel gilding succesfully for many years, doesn't use a light, and may not even own one.

For wikipedia entries, remember that it's a free for all, public input of information site that's not always accurate. "Heat strengthened glass"..... how do you think glass is annealed or tempered? Glass is heated to a specific temperature in an oven, kept at temperature for a specified time and cooled down.

Heat strengthened glass is twice as strong as annealed glass, is not safety glass and does not meet city requirements.

Danny

[Jerry Berg]

Thanks for the very important info on this concerning commercial applications Danny.

I was wondering, For doing a 1'x3' glue chipped and gilded piece that would be just framed or hung indoors for, shall I say "wall jewelry" (is that O.K. Larry?) what type of glass is the glass of choice and why. Specificly, what is the best glass to use for chipping.

Allways Thankful, Jerry

[Danny Baronian]

Jerry,

if you're doing projects for yourself use plate glass, either 3/16" or 1/4". You could use 1/8" (double strength), but you're limited if you want to do any depth carving, and heavy glue chipping could make for some thin areas. Best for chipping would be the former.

A choice Larry would include would be starfire, which is crystal clear. And considerably more expensive. But that's Larry: he's got deeper pockets . And a town of his own.

Danny

[Doug Bernhardt]

Dan, I know I've told you in person but what the heck......that's/was a beautiful piece. One of these days you have to tell me what books/mags/other stuff you use as idea starters. The piece for the Bloomington meet blew me away and am always on the lookout for something a designer of your caliber uses. More to the question at hand.....I've been told by glass shops I go to that any decoration will change the nature of the glass when it comes to cutting at a later date. I've noticed this also while working on "Project Panels" or art pieces where you get to change the size half way through the "Job" ;=(

[Dan Seese]

Hey Doug,

Thank you for the compliment. It was clearly one of my favoirte pieces and I was sad to lose it.

And I have to tell you - it's a bit embarrassing to be asked by you about where I get my ideas. I'm not being coy or promoting a "mutual admiration society" but, frankly, your work is a HUGE inspiration for me. Designing is always a struggle and never just comes naturally. Though I usually try to put my heart into every job, most of my work seems to be fairly unexciting punctuated periodically by some thrilling projects.

One of the things that I've noticed is that we tend to fall into certain cliches in our design, so when I look at a project I try to consider the image that needs to be portrayed and immerse myself in other work of that genre. I eventually set that aside & then go to the drawing board.

On the Rifleworks piece, I think I pretty much had the concept in my mind when I walked away from my first meeting with him. (He originally just wanted PT Barnum letters across the bottom of the glass - I'm glad he changed his mind.) I ran my sketches by Tom Seibert and Bill Hueg and they gave some great great suggestions which helped make it work. I think browsing through Atkinson's Sign Painting book gave me some ideas as that seemed to be somewhat conversant with the period I wanted to portray.

Similarly, with the Bloomington piece you're referring to, when Gary Anderson said "The Stork Club" was one of the buildings that needed a window, I got excited because Art Deco is one of my favorite periods. The basic concept came to mind but I looked at a lot of deco stuff - not really any magazines - before I completed the design. I have a number of art deco books and the Rockerfellar Center is one of the best examples of American Art Deco in existence. I think this subject of how to generate design concepts would be a great topic for a whole new thread.

Jerry, in response to your question about how the rifle was rendered, it was a combination of brush and airbrush. I had someone ask me to make a post about how I did this window. That's something that will take a bit of time, as I don't have digital photos and will need to reconstruct it all in my mind, but I'll plan on getting to this at some point when I'm not so busy. At that time I'll go into more detail about how I painted the rifle.

Dan

[Doug Bernhardt]

Yeo Dan, Surprised to find myself in your list but complimented. The Stork Club impressed me a great deal and I always wondered where you started on that one. It's very elegant and the rendering just right (you know....balance in colour as well as design) Think about posting it so everyone will know what the heck we're talking about. I'm a huge fan of keeping a design "library"...or better..... a place to rip great craftsmen off.

[DAVE SMITH]

Very Nice work Dan. I Like every part of this design ,I remember seeing this years ago.
Dave