Full Chisel Blog

December 1, 2008

Cutting Old Glass

Filed under: Hardware,Historical Material,Of Interest,Proper Tools,Techniques,Uncategorized — Stephen Shepherd @ 7:42 am

My weekend project was to collect enough old glass to glaze the sash doors.  I almost have enough and got more than half of the pieces cut.

I have 40 small pieces of glass that I need to cut to fit up the sash doors on the secretary.  I have selected antique glass for the restoration and that presents certain situations.  For one thing the glass is old, not that that is a bad thing but it is not flat nor is it the same thickness throughout, let alone the contained seeds and grains.  There is also the putty, paint and dirt that must be properly dealt with.


Now I was instructed by a professional Art Glass Master, with a speciality in Glass Restoration, and his name I will not mention as he probably doesn’t want this type of association.  He instructed me that the best glass cutter has carbide wheels which should be replaced on a regular basis.  He said that the cutter should be lubricated (the only instruction I followed).  Cut one continuous cut and don’t go back over the cut.  He uses a marking pen to mark cutting lines on his glass.  He also said that it was important that the glass be clean with all surface accumulations removed.  This can be easily done with a sharp blade such as a single edge razor blade.

Some of his instructions and recommendations are simple things that we may not think of, such as having a flat surface to work on, keeping it clean, throw the scraps away immediately, don’t brush the work surface with your hand, use a bench brush and keep some bandages close.

Well as you may be able to tell, I do it a different way.  In the first place I own currently (I have sold some nice old ones) a fine diamond glass cutter.  It has a 1/4 carat industrial diamond set in a copper bezel set in an iron head on a brass ferrule and birch handle.  I do use oil to lubricate every cut and that little piece of paper towel has a few drops of light machine oil, but bear fat or lard work fine as well.

And I don’t bother cleaning the glass unless I can’t lay a straight edge on the glass and only clean off glass that actually survives the cutting process.  No reason to clean of glass that is going in the garbage.  And because the marking pen didn’t exist in the nineteenth century I used a stained glass technique of making a cartoon (full size drawing) of the glass I need on some white paper.


I place the glass over the drawing of the size I need, position the straight edge so it is offset the distance of the diamond point is from the edge of the iron head.  Get comfortable with the offset of the diamond and it becomes easy to just visualize the proper distance, then hold the straight edge down tight and make the cut from the far end to the near.  I am going to try to cut some freehand without a straight edge, I have seen this done on smaller pieces.

Sometimes the cut doesn’t start just at the far end but cuts to the end just fine.  I was told not to try and finish the cut, but I didn’t listen.  Instead, I reverse the glass, register the point of the diamond in the existing cut, just like placing a chisel in a gauge line, and continue the small cut off the edge of the glass.


After the cut is made the glass is reversed and it is struck with the end of the cutter on the opposite side of the cut.

I tried tapping it with the iron head with its rounded edges and it did work but took some time.  So I tried turning the glass cutter around and used the round wooden end to do the fracturing.  I was amazed how easy this process worked.


On these small pieces a tap or two and the waste just pops off.  Sometimes you end up with a cross fracture or wayward crack that ruins the piece or that runs out requiring some nipping with a pair of pliers



Here is what the fracture looks like if it doesn’t go completely across the piece.  The fractures usually start at one end, but that can start in the middle of the piece, the unusual characteristics of glass.  Watching the fracture move along is fun, until it runs out and ruins the piece.

I cut 24 pieces and still need to cut another 16 pieces.  I ruined about a half a dozen pieces, about half of which I could get smaller pieces from.  I start out cutting the larger panes then cut the smaller.  But there is more waste when cutting antique glass, modern single and double strength glass being more uniform in nature usually get better results.

Once I have them cut then I will use a flat chisel to remove the putty and paint that is still remaining, clean with alcohol then vinegar.  The glass in the sash doors will be held in with triangular points and glazier’s putty.



  1. A glass cutter taught me to use paint thinner – or turpentine – not oil. When I asked why, he said, “It just works better.” I’ve never tried oil, so I can’t compare.

    Is it true that antique glass is thicker at one end because glass flows downhill over time?

    Comment by Joe Cottonwood — December 1, 2008 @ 10:41 am

  2. Great tutorial, Stephen. I’m somewhat surprised that you can glaze a window using 40 disparate pieces of old glass. It would seem that “matching” would be a problem. Also, what is the function of the squarish block on the cutter?

    Cheers — Larry

    Comment by Larry Marshall — December 1, 2008 @ 12:10 pm

  3. One thing I learned about cutting glass is that the faster you snap it after the cut, the better.

    My understanding is that glass doesn’t actually flow downhill. This whole ‘super-cooled fluid’ thing you hear bandied about now and then doesn’t hold up to the ‘sniff test’. If it was soft enough to flow then it wouldn’t be brittle enough to shatter.


    Comment by Metalworker Mike — December 1, 2008 @ 4:34 pm

  4. Joe,

    Thanks for your comments I am going to try turpentine as the oil/lard/fat thing is a bit messy. As for the semi-liquid nature of glass I will answer that.


    There will be some difference, very slight in color, but overall the match should be good enough that no one will notice, they are mostly small pieces.


    Interesting idea about snapping the glass soon after the cut, never heard that one before.

    Now for the semi-liquid state of glass. I have removed a large number of pieces of glass from old sash frames from old houses. In cases where the glass is over 100 years is definitely thicker on the lower end. I have even removed glass from 4 separate window frames that had actually ‘de-glazed’ from the top end of the sash. On two of the pieces the glazer’s putty had stayed attached to the glass, the other two it came out. Now this glass wasn’t too short in the first place as there was still glazing points under the putty.

    Also if you look at old Greek or Roman glass bottles the necks have slumped into the bottle causing an odd distortion of the bottles.

    Comment by Stephen Shepherd — December 2, 2008 @ 9:00 am

  5. Hi Stephen; as to the “semi-liquid” nature of glass, in a class on glass properties that I took in my university materials science program about 20 years ago, the professor demonstrated through use of various thermodynamics diagrams and calculations that it’s physically impossible for solidified glass to flow without being reheated. He should know, although there does seem to be anecdotal evidence to suggest that he was wrong… Assuming historical manufacturing processes resulted in glass that wasn’t of a uniform thickness, I wonder if glaziers set the panes so that the thicker ends were at the bottom to begin with, and that this is what you’re seeing.

    Comment by Steve Wyrick — December 2, 2008 @ 10:50 am

  6. Steve Wyrick — of Scottish fiddle and dance? — is that you? How are ye?

    While proving something is physically impossible is satisfying, it can come back to haunt you. I remember plenty of lectures I gave while teaching math about the golden ratio and violin design. Wish I could take some of it back, though not all. I was presenting the information as I understood it at the time, and that’s the best you can expect of anyone.

    I’ve heard the flowing-glass story from many sources. I don’t know what the variation is — top to bottom. Is it something that could be easily detected by a person setting the panes. If not, then efficiency would dictate that they wouldn’t bother. Also, contrariness would dictate that someone, somewhere, would think that the thick part goes on the top, or the left side, or right.

    It would be fairly easy to measure thickness in existing old buildings — maybe using something like a Hacklinger gauge used in musical instrument measurements. If there is a preponderance of fat-bottomed panes, then the impossibility calculation would have to be revised. If not, then we can do away with the flowing-glass stories.


    Comment by Ken Pollard — December 2, 2008 @ 6:40 pm

  7. Here’s a link for you from the Corning Museum of Glass:



    Comment by Metalworker Mike — December 3, 2008 @ 5:41 am

  8. Steve,

    Thanks for posting your comment and this got me asking questions as did M.Mike’s comment and link, but I am still considering this issue.


    If those gauges weren’t so expensive I would like to own one. My experience with the sagging glass could also be attributed to movement of the sash or settling of the putty, but the glass is still thicker on the bottom of panes I have removed. Glass is indeed an unusual material.


    I read the link, his comparison to lead was weak but I remain skeptical because of the unique characteristics of glass can’t be compared to other materials. (This reminds me of the controversy of seeing stars during the daytime from the bottom of a well).

    As for everyone’s suggestions, I did find that turpentine works better, I do snap larger pieces at the edge of my bench and I am now snapping the glass Immediately after cutting. Lo and behold, the work went faster and all of the breaks were good.


    Comment by Stephen Shepherd — December 3, 2008 @ 7:00 am

  9. I’ve been doing some research on this, and have run into a few good points, the best of which is this:
    There are antique telescopes out there. If glass was able to sag enough to be visible to the naked eye in a pane of glass after 150 years, then a 200 year old telescope should be absolutely unusable. This is not the case. Telescopes, watch lenses, and other things like that are no worse after 200 years than one would expect they were in the beginning. Also, if glass sags then ALL glass should sag. Finding one bottle in 1000 with a slumped top indicates, to me, a ‘manufacturing second’, not an indication that glass sags. The average bottle 300 years old should be noticeably more wonky than the average 150 year old bottle, yet this is not the case.


    Comment by Metalworker Mike — December 4, 2008 @ 5:28 am

  10. M.Mike,

    Good research, however there are a couple of problems. Optical glass such as lenses are very thick, unlike the very thin sheets of window glass in a vertical orientation. And most all early watch glasses were crystal, made from rock crystal as were some early telescope lens.

    Bottles are usually much thicker, so the movement is much slower. Old Greek and Roman bottles appear to have sag in the place where the neck comes from the bottle. This could be explained by being in a fire, so there is some contention.

    You have some good arguments and thanks for the comments.


    Comment by Stephen Shepherd — December 4, 2008 @ 9:30 am

  11. I really have no opinion on whether glass flows or not. I’m guessing that you, M.Mike, have done some good research on glass, but I think there is a problem with your examples of optics and watch lenses, as well as with Stephen’s old bottles. They are very different than window glass in that they are not held constant with respect to the gravitational field.


    Comment by Ken Pollard — December 4, 2008 @ 3:45 pm

  12. Interesting comments on glass.

    I am looking for some advice. I have a very large mirror in a bathroom that I am updating. The mirror has a small rectangle cut into it to accomodate an electrical outlet. I need to update the outlet to a ground-fault-type outlet. The problem is that the person who cut the rectangle in the mirror cut it too small and it is not possible to remove the old outlet though the existing hole. I could simply remove the mirror and get access to the outlet and change it, but the mirror is quite large and I don’t trust my helpers to help me remove it confidently without breaking the mirror. The easy solution is to enlarge the existing hole–I guess I need to make the hole about 3/16 of an inch longer than it is currently. Can I slowly/gently grind the hole a little bigger by using a Dremel tool and grinding stone? Any advice appreciated.

    Comment by David — January 1, 2009 @ 12:06 am

  13. David,

    I would imagine if you have the correct grinder you can grind away the mirror. I am not sure how to advise you as it is out of my field of expertise, although I have seen it done. One bit of advise is to protect yourself from the nasty dust of grinding glass.


    Comment by Stephen Shepherd — January 1, 2009 @ 11:46 am

  14. You need a mirror nibbler tool. http://www.crlaurence.com/ProductPages/M/MN1750_2670.html?Origin=
    You can “nibble” the opening without breaking the mirror. Think of taking small “bites” out of the edge of the glass. Very simple to use, no experience required.

    Comment by Rick Nelson — August 9, 2009 @ 6:10 pm

  15. That nibbler is a bit too modern for me. I have used glass plyers for that purpose and some glass cutters (not mine) have those slots for nibbling. I could probably use a saw wrest for the same purpose.

    Thanks for your comment.


    Comment by Stephen Shepherd — August 11, 2009 @ 7:33 am

  16. I need advice on how to cut a curved vinatage cabinet glass pane please.?

    Comment by Japie de jongh — August 28, 2012 @ 3:37 pm

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