To test glass for internal stress, place it between two polarising filters turned 90 degrees to each other and look through both against a bright diffuse light. Well-annealed glass shows no colour and stays uniformly dark; coloured bands or patterns mean internal stress and the piece should be re-annealed. A re-anneal is safe on shape, because the 510 °C hold of soda-lime sits 190 °C below its 700 °C softening point.
Part of Glass Blowing and Forming
Detecting locked-in stress in finished glass with polarised light and deciding what to do about it.
Stressed Glass Rotates Polarised Light And Shows It As Colour
Glass under stress becomes birefringent: it passes light differently along different directions. Light polarised by the first filter is altered by the glass, so some of it can pass the second filter, which is set to block unaltered light. Where the glass is unstrained nothing gets through and the field stays dark. Where there is stress, colour appears. The corpus’s Glass Blank Prep article describes the same effect for lens blanks: stressed regions appear bright.
What You Need
- Two polarising filters. Salvaged polarising sunglass lenses or the polarising film from a salvaged flat display are the likeliest sources (recalled, not from the corpus). Use the two from the same pair if possible.
- A bright, even, diffuse light: daylight sky, or a lamp behind frosted glass or paper. A direct flame gives uneven illumination.
- A dark background and a steady stand.
- A marker to note where the colour appears.
Method: The Test
Step 1: Set the filters 90 degrees apart. With nothing between them the field should go dark; turn one until it does. This is the crossed position the carrier describes.
Step 2: Place the glass between them and look through.
Step 3: Rotate the piece slowly. Some strain patterns appear only at certain angles.
Step 4: Read the result. Uniform darkness means little stress. Coloured bands or patterns mean stress; the carrier’s rule is to re-anneal.
Step 5: Note where the colour is. Concentrations at the pontil scar, the base, the neck or a joint tell you where the stress was locked in; see Joining and Sealing.
Step 6: Re-anneal. Return the piece to the lehr, heat it up slowly, hold at the annealing point and cool slower than before; see Annealing Oven.
Step 7: Test again. A second failure means the oven schedule is the problem, not the piece.
Without filters, the corpus’s glassblowing article suggests tapping: a clear ring indicates proper annealing and a dull thud suggests residual stress. Treat that as a weak screen only.
Worked Example
Why a re-anneal does not slump the piece: soda-lime softens at 700 °C and is held at 510 °C, so the hold sits 700 − 510 = 190 °C below the point at which the glass starts to sag under its own weight. Borosilicate has 820 − 560 = 260 °C of margin and lead glass 630 − 430 = 200 °C, from the carrier’s table. The oven must still be heated slowly, since a cold thick piece cracks if it is heated fast; the corpus gives no heating rate.
Common Mistakes
- Testing a hot piece. Test cool glass; stress changes with temperature.
- Uncrossed filters. If the field is not dark with nothing between them, the test shows nothing.
- Reading colour from window glass edges or reflections instead of through the piece.
Safety
Strained glass can burst when handled
A piece with strong residual stress can fracture violently with a light knock or a change in temperature, throwing sharp fragments. Handle a suspect piece with gloves and eye protection, do not test it by thermal shock, and do not fill it with liquid until it has passed.
Hot glass does not look hot
A re-annealed piece that has left the oven looks cold below about 600 °C long before it is safe to hold. Wait until the oven is at room temperature.
What’s Next
- Learn next: Annealing Oven (Lehr) — fixing the schedule
- Combine with: Pressing Lens Blanks — screening every blank