Cutting Tempered Glass

topic posted Mon, May 19, 2008 - 12:56 PM by  Richard
I have a 19" x 52" piece of tempered glass salvaged from an old shower door that I would like to cut down to 47" for a project. the glass has a bit of a pebbled texture to it as well.
I have a good quality glass cutter and breaking pliers for tools, but I've only worked with non-tempered glass previously.

Is breaking tempered glass as simple as "score and break", with the scoring being done on the smoothest side? Any other tricks of the trade I need to know?

In glass breaking, one usually only gets one chance to get it right!
posted by:
  • Unsu...

    Re: Cutting Tempered Glass

    Mon, May 19, 2008 - 3:09 PM
    as far as i know its impossible to cut. read this to know why

    btw, tempered is the same as toughened

    Toughened glass must be cut to size or pressed to shape before toughening and cannot be re-worked once toughened. Polishing the edges or drilling holes in the glass is carried out before the toughening process starts. Due to the balanced stresses in the glass, damage to the glass will eventually result in the glass shattering into thumbnail sized pieces. The glass is most susceptible to breakage due to damage to the edge of the glass where the tensile stress is the greatest, but shattering can also occur in the event of a hard impact in the middle of the glass pane or if the impact is concentrated (for example, striking the glass with a point). Using toughened glass can pose a security risk in some situations due to the tendency of the glass to shatter completely upon hard impact rather than leaving shards in the windowsill.
    • Re: Cutting Tempered Glass

      Wed, February 9, 2011 - 6:02 PM
      Well actually you can cut tempered glass but you have to have a special laser cutter used by experts for this. First you need to anneal the glass to approximately 900 degrees F, and then let it cool for about eight hours. Dip the cutter into kerosene for a nicer cut and then start with the cutting, very slowly!

      I hope that some of this was helpful to you. Good luck with the cutting!

      • Re: Cutting Tempered Glass

        Thu, February 10, 2011 - 7:43 AM
        Can't somebody do something about this twit ? A cut glass....realy anneal it and waie 8 hours...then you can contact his floring o. and tell them how full of shit they are SPAMMMMMMMM
        • Re: Cutting Tempered Glass

          Sat, December 31, 2011 - 3:30 PM
          The only "twits" I've ever known are those who get rude with others because they think they know more than they do. Conventional (inside the box) wisdom is exactly what Brian posted. It's a process of "untempering" the glass with the same high heat used to temper it. The cool down period is necessary to avoid stressing the glass again.
          The laser method is very real and uses a high power laser that will set you back some serious bucks. It is not your 5 milliwatt toy.
          Now, if you're going to talk with the grownups, mind your manners.
  • Re: Cutting Tempered Glass

    Mon, May 19, 2008 - 5:01 PM
    Actually you can "cut" it, and I have helped do it. Expect about 30% (or more) breakage...

    BTW, you could always wet grind it off...

    The trick is to de-temper it. And the method I learned from an admittedly crazy glazier was to put the work on some dowels on a table so its not touching the work surface, soak a thick string in Naptha or gasoline, place it where you want the cut and light the bad boy. Have an extinguisher handy. When the flame goes out, pull the cut site to the edge of the table, remove the dowels and while putting downward pressure on the piece you are removing, have somebody smack it with a rubber mallet. You will get 1 of 2 things... A good edge, or a pile of shards...

    Does NOT work on Laminated glass!!!

    Fire extinguisher
    Eye protection

    Be careful...
    • Re: Cutting Tempered Glass

      Mon, May 19, 2008 - 5:21 PM
      You know, I haven't met too many glaziers who weren't crazy.........
      • Re: Cutting Tempered Glass

        Mon, May 19, 2008 - 9:26 PM
        Wasn't the trick to lightly score the cut line first on Both Sides ,because it is after all two seperate sheets of glass with plastic sandwiched between, ThEn do the flaming string thing. Then there were two versions, one like you said to snap it off normally over the dowel, and the other is the quick plunge in cold water. The quick hot-cold will fracture the material on the score line. I know for sure this'll work on cutting beer bottles if you want to make glasses out of them. But you only score the outside before the flaming string when doing bottles.

        Then there was the one about being able to cut glass with shears, if you held the work under water?? Never could get that to happen.

        Don't hold me to this. i've heard many versions of the story. Some may be urban legends. But if you have a free aturday afternoon and a bunch of safety glass pieces, you could try out different things until something works out.
        • Re: Cutting Tempered Glass

          Tue, May 20, 2008 - 7:51 AM
          Your'e thinking of safty glass, the type used in auto windshields, and security windows, not tempered glas, the type used in the other auto windows.
          I've heard of the hot wire method, never tried it. Way to much trouble, with a very low rate of sucess, for a first time effert.
        • Re: Cutting Tempered Glass

          Tue, May 20, 2008 - 3:51 PM
          The guy that taught me this did not score the glass... Surprised the hell out of me because I thought the same thing.

          I have a real good self oiling cutter and have done some big pieces in my time, including 1/2" thick plate. Never done tempered glass other than with Mark.
          • Re: Cutting Tempered Glass

            Tue, May 20, 2008 - 5:41 PM
            If ya score or crack it at all even after heating it. it can shatter. When I saw it done it was with a hot wire he heated it for a while then whacked it. For my money I just order tempered glass to the size I need. It really is not that much.

            • Re: Cutting Tempered Glass

              Wed, May 21, 2008 - 4:48 AM
              I've had a number of sheets of tempered glass shatter on me from just a little shock to the edge of the piece. It is disconcerting feeling to have a sheet shatter into tiny fragments when carrying it (!). I can hardly imagine cutting the stuff. I would buy a piece that is the right size too, and i'm very much a cheapskate diy kinda guy.
          • Unsu...

            Re: Cutting Tempered Glass

            Wed, May 21, 2008 - 6:57 AM
            *************I have a real good self oiling cutter**************

            What does oiling the cut do?
            • Re: Cutting Tempered Glass

              Wed, May 21, 2008 - 9:46 AM
              As I understand the situation, it's more about keeping the cutter wheel rotating smoothly than oiling the cut. You're trying to achieve an even, consistant score line without any skips.

              You're also dealing with microscopic pieces of material that are quite abrasive so you want to suspend the little particles of glass in a lubricant so that they dont get caught up in the moving parts of the tool.
              • Re: Cutting Tempered Glass

                Wed, May 21, 2008 - 9:49 AM
                BTW my solution is to redesign te project to use the glass size available. Maybe I will try the heated wire trick some other time when I have a cast off that I don't have any idea what I want to do with.
            • Re: Cutting Tempered Glass

              Fri, May 23, 2008 - 11:46 AM
              The cutting oil cools the score and sort of fills in the line. As glass is still a partial liquid, it wan't to heal the score line, making it harder to break.
              • Re: Cutting Tempered Glass

                Fri, May 23, 2008 - 11:58 AM
                that was supposed to say wants to heal.
                And you can use "glass cutting oil" from a glass supplier, sewing machine oil or 3-in-1 oil. Has to be thin and light. No vegie oil! :)

                Anything done to modify glass has to be cooled. Saws and grinders have water feeds, cutters have oil and drills should have water spritzed on the entry point. Getting any kind of glass too hot will cause breaks, and it won't always break where you want it to.

                Sorry about all the posts/soap-boxing.
                I come from a glass family and work in a stained glass shop. :)
        • Re: Cutting Tempered Glass

          Thu, May 12, 2011 - 5:55 AM
          The hot / cold method is the only way i've ever heard of working as Briggi described, its also supposed to be good for cutting bottles down into drinking glasses.
      • Tim
        offline 0

        Re: Cutting Tempered Glass

        Fri, August 2, 2013 - 7:57 AM
        My two paragraphs worth,

        I use Transmission fluid for lubricant super cheap buy it by the quart.

        When cutting laminated glass,
        PPE required maybe go outside first...
        score each side apply just enough pressure for the score line to crack. Cover both sides in paint thinner, use a small amount flip the glass then use a enough that it spills over the sides along the score line. Light the small pool of thinner on top and of course the entire line will engulf and the flame will follow along under the piece too. Even though all your thinner hit the ground upon flipping the glass over enough stays.

        As soon as you light the fire start to wiggle slightly the lamination layer will sag along the cut. Sometimes the lamination will tear away from itself at this point if so let it. If the fire doesn't go out by now blow it out. Take a new razor blade why risk it with an older blade right... Run your razor blade along the lamination between the two sheets keeping the blade off the glass to cut it and...

        Presto change O two pieces of laminated glass.

        The hardest part about cutting laminated glass, even a windshield, is cutting the two sheets along the same plane perpendicular to the glass. With one line parallel to the other, or put another way directly atop the other, the rest is easy.

        Some was said about windshields being laminated and the rest of a vehicles windows tempered safety glass, this is increasingly not the case and you don't have to drive a BMW to find laminated door glasses. Many companies have expiremmented with laminated door glasses and back glasses as well, Our theory was this design prevented theft and burglary while trapping you inside should you roll over the side of a hill and end up underwater.
        If your door glass seems to have two bevels instead of one its laminated. BMW goes above and beyond of course their laminated door glass is half an inch thick or more and is also a smaller thicker piece of glass laminated to a regular door glass.
        I've installed over 4500 door glasses, I spent over 5 yrs working six days a week 12 to 16 hours a day installing an average of 50 auto glass pieces a week.
        I also did my fair share of flat glass cutting and I learned from an old hippie. He never went anywhere without tranny fluid and thinner, He forgot the glass several times... Never the tools though.
        Sorry if i was rambling that's what crazy people do....
  • Re: Cutting Tempered Glass

    Fri, May 23, 2008 - 11:52 AM
    You cannot cut tempered glass.

    The ideas given here may work, but my father has owned a glass shop for 30 years and has never has any success cutting tempered glass.
    The tension in the glass that causes it to be safer (ie not leaving shards in the sill for a burgler to cut themselves on and sue you) also causes it to do it's little shatter dance with any modifications cutting, grinding or drilling.

    Laminated glass (two sheets or glass fused together with plastic between them) can be cut: score bothe sides of the glass, run the break with runners and slide an exacto between them to cut the platic.
    • Re: Cutting Tempered Glass

      Tue, May 27, 2008 - 8:12 PM
      you can cut tempered glass.. its not very easy... people in the aquarium hobby do it all the time to put stand pipes into their tanks... they drill holes... which is what your basically doing when cutting lol...

      What you will need... diamond blade.. time.. lots and lots of water... and go REAL slow or the whole thing will break...

      • Re: Cutting Tempered Glass

        Wed, May 28, 2008 - 8:18 AM
        Tanks are not tempered. They are thick glass but not tempered.

        • Re: Cutting Tempered Glass

          Fri, May 30, 2008 - 10:13 PM
          Thanks JSin :)

          Annealed glass has been cooled slowly, leaving less tension, making it cuttable.
          Tempered glass is cooled in a different way, leaving lots of tension in the glass, which means the glass will break with any trauma, including a glass cutter, hammer or drill.

          Sorry to be so bullheaded about this, but my family has been in glass for about 60 years :)
          • Re: Cutting Tempered Glass

            Sat, May 31, 2008 - 7:56 AM
            You CAN anneal the glass, but then you don't have tempered glass anymore. You would have to re-temper it, and that's a more complex opperation than annealing.
        • Re: Cutting Tempered Glass

          Mon, June 2, 2008 - 5:40 AM
          sorry to say this but no... the tank glass used in most modern aquariums..allglass and the ones done in dallas are tempered glass.... not just thick. The all glass aquarium sometimes are made of tempered but sapphire aquariums are last time I checked with the manufacturer. All of the table tanks you see are made of tempered as well.

          • Re: Cutting Tempered Glass

            Mon, June 2, 2008 - 9:24 AM
            Well this must be a very new trend since the ones I have kept and the ones I sold including the shard I had pass through my hand on a broken one were just basic glass. I have kept over 75 tanks including at one point a tank room with over 1500 gallons in tanks.

            Ya know when ya see a cracked tank. Tempered Glass does not crack. It shatters. In fact the piece of glass that punctured my hand was from an "All Glass brand tank".

            Actually reading most of the stuff I could find on the web the only "AllGlass" tanks that are tempered are over 200 gallons and need to be ordered with filter holes pre drilled. Tempered tanks also have a logo stating they are tempered so you do not make the mistake of trying to drill them.

            Sorry chris you are mistaken.

            • Tim
              offline 0

              Re: Cutting Tempered Glass

              Sat, October 18, 2014 - 5:27 AM
              I used to build custom aquariums. I started out a tank enthusiast and I had an insane fish in a 150 gallon tank ram a specific corner of the tank repeatedly. The red belly paccu was about 18 inches in lenothing and almost 4 inches thick. Technically I needed a bigger tank any how and that fish was demanding it lol. At the time i worked at a plastic fabrication shop and had access to a vast array of plastics of all types and sizes plus a good CNC machine. I also had the good fortune to be standing in front of the tank when Goliath separated that corner and spilled about 20 gallons of water on the floor. I temporarily clamped the tank shut and ran to work to talk to my boss. That's what got me into making tanks. All aquariums with the exception of most 10 gallon tanks have a tempered glass bottom. Most tanks don't have tempered glass sides until you get to sizes bigger than 150 and even then they usually convert to some kind of polly type of material to cut down on weight. So you're kinda right but still wrong. The tempered bottoms are almost a must have for the amount of weight a tank bottom carries and tempered sides would be a big risk for residential aquariums because people tend to bump into the corners of tanks and tempered glass has a sensitivity to edge trauma as most of us know. I have had a couple hundred tanks over the years most of which I made and sold.
          • Re: Cutting Tempered Glass

            Tue, October 13, 2009 - 8:13 PM
            chris your wrong most aquarium are tempered and come already pre drilled for overflows. now i will say this too some aquaruim only have tempered glass on the bottom and not the sides thats why you see them drill on the sides and not the bottom of the tank. Read on any tank and it usually says right on the bottom of tank DO NOT DRILL TEMPERED GLASS.. all 55 gallons aquaruim are tempered all the way around come look in my back yard i have a few shatters ones

  • Re: Cutting Tempered Glass

    Wed, May 28, 2008 - 8:38 AM
    The wikipedia article on tempered (toughened) glass is very informative.

    It says it cannot be cut or drilled and must be annealed first. It has an article on annealing glass too, but a quick look tells me it's not a good diy project.

    I remember now that the way to tell if glass is tempered is to look at it with polarized sun glasses. You can see the stress patterns caused by the tempering process.
  • Re: Cutting Tempered Glass

    Tue, June 3, 2008 - 6:57 PM
    I cut bottles with a hot wire, not sure if thats tempered glass. The way you do it is to take a steel wire heated with electricity to orange hot, place that against the glass to heat it up, and then cool the heated glass all at once with cool water and tap lightly. I'm guessing the quick temp change causes enough contraction to make the break, which turns out to be very clean. You may need to tap it harder to break it since it sounds thicker than bottles. I think finding a 19" piece of hot wire might be tough though.
    • Re: Cutting Tempered Glass

      Tue, June 3, 2008 - 10:29 PM
      Nope, bottles aren't tempered.........
      They can be cut this way or with a diamond saw blade with a water feed.....
      • Unsu...

        Re: Cutting Tempered Glass

        Thu, June 5, 2008 - 1:52 PM
        but they can be cut with precise Buddhist energy waves. years of practice. i knew a guy in Shimla, India who once did it with fresh green grape leaves and large amounts of prayer flags.
      • Re: Cutting Tempered Glass

        Thu, June 5, 2008 - 10:29 PM
        you can score em and pop em just like with any other glass as well. Problem is getting a precise cut and giving a relief cut to the top or bottom depending on what ya want to keep. The scoring method does not create a super rounded cut though unless you take an oxy acetylene torch to the edge.

        • Re: Cutting Tempered Glass

          Fri, June 6, 2008 - 5:55 AM
          I'd tie a cotton string over the score line, then squirt Ronson on ithe string. Then light it up. Then just as soon as it burned out I'd dip it in the rain barrel. It'd snap apart on its own. I'd take a file/grinder/emery cloth to the sharp edges.

          I'll try torching the edges but maybe hard to control even heating.
          • Re: Cutting Tempered Glass

            Fri, June 6, 2008 - 12:19 PM
            Have you actualy done this ?
            It doesn't seem like a string with a very small amount of flamables would generate enuf heat .
            • Re: Cutting Tempered Glass

              Tue, October 13, 2009 - 10:43 PM
              Sorry Dan I didn't see your question. Hope you don't mind my reply about 1 1/2 years late.

              Yes, I have done this exactly as outlined back in 1979. Got the idea from a 1950s Popular Mechanics magazine.
              Those old magazines are solid gold for DIY from an era when people actually made and fixed things at home.
              Before today's throwaway mindset.

              And yes, I still have a couple of those beer bottle glasses around unbroken as proof.

              Kero (kerosine lamp fuel) works well too in sustaining fire for a while, unlike low flashpoint quick burn gasoline.
              • Re: Cutting Tempered Glass

                Wed, October 14, 2009 - 5:22 AM
                I know the procedure, but that is for un-tempered glass. I don't see how it could be done with tempered.
                • Re: Cutting Tempered Glass

                  Wed, October 14, 2009 - 11:20 AM
                  No practical home solution to that exists, short of putting together a home-made industrial grade tempering oven ( 800 degreesF) to detemper the pane for work, and then turn around to re-temper it.
  • Re: Cutting Tempered Glass

    Sun, October 25, 2009 - 10:30 PM
    no you can't cut tempered glass. give it up.
    • Re: Cutting Tempered Glass

      Fri, June 25, 2010 - 11:07 PM
      Maybe you cant "cut" it but I can "drill" holes in it. As has been stated here by someone else. I have a 90 gallon aquarium that was given to me bc it had no overflow holes cut in it and the bottom said "tempered" in huge letters. After going over it with the guy at my local aquarium store, we decided to try it bc I didnt give a crap if it broke or not. the success rate is VERY low b/c very few people have the patience it takes to spend an hr cutting very slowly through the glass with a very expensive diamond tip drillbit. But I have two corner overflows in this tank sitting not five feet away from me right now and holding up just fine. I believe I'm gonna make a video of us drilling one and speed it up. Then put it on Youtube for all the non believers out there.
      • Bob
        offline 0

        Re: Drilling Tempered Glass

        Thu, July 29, 2010 - 8:45 AM
        Carl - Your tempered aquarium bottom must not have been very "tempered". I work for a glass company and we test a lot of different diamond tools on various glass. All glass is tempered to some degree, however, glass that it heavily tempered can not be drilled or sawed without first removing the temper somehow.

        Tempered glass, like other tempered materials, places the molecules of the material under a stress. While the stress increases the strength of the material, it also acts much like a spring. When a fragile material like glass is drilled or cut, the release of that stress around the cut will cause the glass to crack, if the material is highly tempered. On highly tempered glass, it can be more like an explosion rather than a simple crack.

        The temper of glass varies with the manufacturing process and can vary a lot from glass to glass, often with no specific intent by the manufacturer. We have even seen highly tempered glass that the manufacturer said was not intentionally tempered. We often see that "accidental tempering" in crafted glass like hand blown glass or fused glass. The most consistent highly tempered glass that we have tested is "Pyrex" glass, which we all have for cooking utensils although it is also used for scientific beakers, etc. If you try to drill it with a good diamond drill bit, it will take forever, then about halfway through, it will shatter around the hole. Pyrex is a perfect example of the extreme stress that is released when it is drilled - it literally explodes when you drill it.

        One of the best write-ups we have seen on drilling glass and other materials with diamond drill bits is on the Diamond Drill & Tool site referenced in some other glass posts.

        We also sometimes use their diamond drill bits or diamond saws for our glass drilling or testing and have been very pleased with their products and support.
        • Re: Drilling Tempered Glass

          Wed, December 8, 2010 - 8:14 PM
          Bob is spot on about what he says. I would recommend going to your local glass shop and ordering a piece of textured tempered glass. I know this takes away from the DiY aspect. If not a local glass shop, I recommend going through OldCastle Glass which is based in Denver.
  • Purchase

    Thu, December 2, 2010 - 1:40 AM
    I would suggest you just purchase a small piece for your project. It appears like you are going to loads of trouble in order to save a few dollars.

    • Unsu...

      Re: Purchase

      Thu, December 2, 2010 - 4:09 PM
      ********It appears like you are going to loads of trouble in order to save a few dollars. ***********

      Isn't that the whole point?
      • Re: Purchase

        Wed, December 8, 2010 - 8:47 AM
        Well, obviously. But why do THE IMPOSSIBLE, which usually ends up costing more in the long run?
  • Re: Cutting Tempered Glass

    Tue, May 10, 2011 - 7:43 PM
    yo mad dawg should you just let the string burn till it goes out or let it burn until a certain point then put it out?
    • Custom hot rods with chopped roofs sometimes ned to cut the safety glass of the windshield to get it to fit the new frame size. And this may be similar to tempered glass. The technique they use is light passes with a dremel holding a diamond blade, or a router with a diamond burr, or a tile cutter with a diamond blade. Repeated light passes so the windshield does not heat up. But the most reccommended way to cut a windshield is using the very slow process of sandblasting through with an $80 HarborFreight blaster. Takes all day. There's a youtube vid on it.
      You can get old glass scraps from glaziers and car windshield places to practice on.
      Do a search with keywords Hot Rod Custom top chopping windshield cutting.
  • Sue
    offline 0

    Re: Cutting Tempered Glass

    Mon, September 16, 2013 - 10:56 AM
    I cut tempered glass often. It needs to be un tempered first by heating it slowly up to 1000 degrees in a kiln. Heat at about 200 degrees an hour, hold tempurture at 1000 degrees for 30 minutes, then cool at 200 degrees an hour. Remove when cool. Score it like any other glass and break with glass pliers by lining up center of plier with score mark. It will snap straight ( hopefully).
    If this sounds complicated, just ask a person that works with fused glass or works with a kiln to cook glass.
    The glass is no longer tempored and will break like any other glass.
    Myscal Glass and Jewelry

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