EZGlassPattern guideWhich cuts are impossible in glass?

Glass breaks along a score from edge to edge, and it wants to break straight. Everything below follows from that.

Answer in 30 seconds

You can't cut a sharp inside corner or a piece completely enclosed by another piece. Deep inside curves, long thin slivers, crowded junctions and lines that don't close are not strictly impossible but usually break or fail. Most are fixed with one extra line or by merging pieces.

Impossible: sharp inside corners, pieces inside pieces.
Risky: deep inside curves, slivers and thin necks, crowded junctions.
A drawing mistake: lines that stop short of another line.

Impossible and risky cuts, and the usual fix

Shape	Why it fails	Usual fix
Sharp inside corner	A score can't turn inward; the break runs on past the corner	Add a line through the corner
Piece inside a piece	The cut would have to start and end inside the glass	Add a line from the inner piece out to an edge
Deep inside curve	The glass in the bay chips or cracks across	Add a break line, or groze it out in stages
Sliver or thin neck	Snaps on the break or on the grinder	Merge into the neighbour, or widen it
Tiny piece	Too small to hold safely while grinding	Merge into the neighbour
Crowded junction	Many points meet in one spot; weak and hard to fit	Move lines apart or merge pieces
Open seam	A line that stops leaves a piece that can't be cut out	Extend it to the nearest line

Key takeaways

Scores run edge to edge — there's no way to start or stop a break in the middle of a sheet.
Glass breaks straight — curves are possible, but the tighter and deeper, the riskier.
Inside corners need a line — turn one impossible piece into two easy ones.
Size changes the answer — check slivers at the finished size, after the foil or lead takes its share.

Check a pattern, or make one from a photo.

Sharp inside corners

An outside corner, like the point of a diamond, is easy: two straight scores. An inside corner, where the edge turns back into the piece, can't be scored in one go, because the break wants to keep running straight past the corner. The fix is a line through the corner, so the piece becomes two.

Deep inside curves

Gentle inside curves are fine. A deep, narrow bay (deeper than it is wide) is where glass cracks across. Experienced cutters score a series of shallower curves and nibble the waste out with grozing pliers in stages, then grind. For a beginner, a break line across the bay is usually the better answer.

Pieces inside pieces

If one piece sits completely inside another, like the middle of a letter O, the outer piece would need a hole cut in it. Hand cutting can't do that. Add a line from the inner piece to the outer edge, which splits the outer piece into one or more pieces that can each be cut.

Slivers, thin necks and tiny pieces

Remember the line takes width: about 1 mm per line for copper foil and about 2 mm for lead came. A strip that measures 4 mm on the drawing can end up only 2 to 3 mm of glass. Thin strips snap; tiny pieces are hard to hold against the grinder. Merge them or widen them. See copper foil vs lead came.

Crowded junctions and open seams

Where five or six lines meet at one point, the pieces all come to sharp points, which are fragile and hard to fit. Junctions that sit very close together have the same problem. And a line that stops short of another line leaves a piece that isn't closed, so it can't be cut out.

The free cut checker finds all of these at your finished size. The limits it uses are starting values, not a standard (the Beginner, Intermediate and Advanced settings make them stricter or looser): experienced cutters can manage more than beginners.

Step by step

1
Set the finished size
Work at the real size of the panel, and allow about 1 mm per line for foil or 2 mm for lead came.
2
Find inside corners
Look for edges that turn back into a piece and add a line through each corner.
3
Find enclosed pieces
Any piece completely inside another needs a line out to the edge.
4
Check curves
Split any inside curve that is deeper than it is wide, unless you are confident with grozing pliers.
5
Check thin and tiny pieces
Merge or widen slivers, thin necks and pieces too small to grind.
6
Check junctions and lines
Spread out crowded junctions and close any line that stops short.

Frequently asked questions

Can you cut an inside corner in glass?

Not in one piece. The break keeps running past the corner. Add a line through the corner so it becomes two pieces.

How deep can an inside curve be?

It depends on the glass and your skill. As a rule of thumb, be careful with any curve deeper than it is wide; those usually need grozing in stages or a break line.

Can you cut a hole in a piece of glass by hand?

No. A piece completely surrounded by another needs a line from it to the outer edge, so the surrounding glass becomes cuttable pieces.

How thin can a piece of stained glass be?

There's no single number; it depends on the glass, the method and the length. Check thin pieces at the finished size after the foil or lead allowance, and widen anything that looks fragile.

Why do my pieces crack when I grind them?

Usually because they're too thin, have a deep inside curve, or come to a fine point. Those shapes are worth fixing in the pattern.

Is there a tool that checks this automatically?

Yes. EZGlassPattern's free checker flags each of these at your finished size and suggests a fix. It runs in your browser.

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