Step 6: Cutting the Glass

Cutting the Glass

Getting the glass the right size is a little tricky. The thing with cutting glass is that you don’t actually “cut” it, you just scratch a line in the surface, and then “snap” the glass. If you did a good job scoring the line, you’ll get a good, clean break in the glass. If you screw up the size, however, you could be in trouble.

If you cut the glass a little smaller than you expected, you may still be able to use it, provided that you didn’t cut it too short. Test-fit the glass in the frame and see if you can slide the glass such that you can see the edge of the glass from the front of the frame. If so, then you’ll need to cut a new piece.

Cutting the glass slighly too big is even worse. It is very hard to cut a narrow (less than an inch wide) strip of glass, so if your glass is a fraction of an inch too big, there’s not much you can do about it to get it to fit. So my best advice regarding the glass is to make sure you get the size as close to ideal as possible.

The way I do this is to cut the mounting board for the frame first. The mounting board is the board to which the artwork will be affixed, and the mats laid on top. It’s not an actual “board”, but rather a stiff paper or foam sheet. The mounting board is the rear-most panel of the frame. I buy mine in big sheets at a local art supply store (Wallack’s again). Look for the “acid-free” kind to protect your artwork.

Measuring the recess in the back of the frame
First, I measure the inside dimensions of the back of the frame on top of the rabbets (that is, the recess in which the mounting board is going to sit) using a steel ruler because I find it more accurate than a measuring tape. However, for larger frames (like this one), my ruler isn’t long enough (it’s only 18 inches), so I have to use a measuring tape. I leave a couple millimeter gap to ensure the glass will fit easily.
I then transfer the measurements to the mounting board, and use a strong pair of scissors to rough-cut the mounting board. I cut about 4-6 inches outside the lines I drew. Then I place the piece in my mat-cutting guide and make the precise cuts using the straight-cutter that came with the kit. I use the mounting board as a template for cutting the glass.

Marking lines on the mounting board

Then trim to the exact size using the mat cutter
I then lay a sheet of glass on my padded table (covered with plastic sheeting to protect it from shards of glass). I buy my glass in sheets 2mm thick and 36″ x 36″ square. They cost me about $15 per sheet, and I buy them at a local glass and mirror store (Centennial Glass, in case any of you are in Ottawa, Ontario).
Lay the mounting board in one corner of the glass, then line up a drywall T-square, checking with the glass cutting tool to see where the cutting wheel will end up. I’m actually using a steel ruler instead of a drywall square in the photo, and I’m just making a practice cut on a small piece.

Next, holding the tool perfectly vertically and applying moderate pressure, run the cutter along the edge of the square. It should make a constant scratching noise all the way along.

Scoring the glass
When you get to the end, lift the glass up and apply light bending pressure along the score line. You need to do this within a minute or two of finishing your score line, as the glass will actually start to “heal” itself. The glass should snap cleanly along the line. Always wear work gloves when doing working with glass!

Snap the glass along the score line
This is actually pretty tricky. If your cutting tool “skips” at any point during scoring, you will almost certainly not get a good break in the glass. That is, the glass will break along the consistent part of the line, but when it gets to the gap in the score line, the break will veer off in another direction, ruining the glass. It takes some practice to get this right. Make sure you’re using a good cutting wheel, and oil it periodically with WD-40 or clean oil.

To be perfectly honest with you, I actually get the glass shop to do the longer cuts for me. I’ve ruined a few too many pieces of 36″ x 36″ glass trying to cut glass for large projects, so now I leave the big cuts to the pros. That way if they ruin it, they pay for to replace it and try again, instead of me. I still do the smaller cuts (up to 16″ or so) myself, but for cutting across the whole sheet (36″), I try to avoid it if I can. It’s just too stressful.

When I go to pick up the glass for a project, I bring the mounting board with me, already cut to the size I want. I give them the board and tell them I want a 36″ x 36″ piece of 2mm glass, with a piece cut out perfectly matching the mounting board. Some places may charge an extra couple of dollars to make the cuts, but it’s a lot cheaper than paying to replace a $15 sheet of glass that you ruined because your cutting wheel didn’t make a perfectly continuous score line.