Notes on Polarised Light Filters

Polarised light filters are used to detect stress in a non-destructive testing method in kilnforming.  The use of the filters is described in this blog. To produce consistent reliable results, there are certain conditions.

 The light source needs to be diffused in such a way that it is even across the viewing area.  An intense, single point light makes it difficult to determine the relative intensity of apparent stress. Another tip is that you can use your phone or tablet as a source of diffused light and as the bottom filter.  It emits polarised light, meaning only a top filter is needed.

Stress halos from broken and fused bottles

 It is important that the glass being tested is of the same temperature throughout to get a meaningful result.  This was emphasised to me when I was running a series of tests. I got in a hurry to test for stress to be able to start the next trial quickly.  I began to notice inconsistencies in the amount of stress I recorded for results of the series of tests.  Going back to the stressed test pieces, showed different stress levels when they were cold from when they were warm.

 The conclusion is that the glass to be tested for stress must be the same temperature throughout.  Even if it is only slightly warm, the apparent stress will be exaggerated.  It may be that the testing can only be done 24 hours after removed from the kiln.

 Stress will be more evident at points and corners.  The light will be brighter at highly stressed points, and even at extreme stress exhibit a rainbow effect.  More generalised stress is evident in a lighter halo.

Stress points in a drawing square illustrating the concentrated stress at corners

 It is much more difficult to check for stress in opaque areas of a piece.  If there are transparent areas, the stress will show there, although the stress may originate in the opaque ones. To be aware of potential stress in the combination of opaque glass, strip tests must be conducted on samples of the glasses. 

 Remember to include an annealing test too, as the stress test does not distinguish the type of stress.  If the annealing test shows stress, the annealing was inadequate. It is of course, possible that the glass is stressed because of incompatibility.  But the only way to determine that is to fire another test with a longer soak at annealing.

Containing Stress

People frequently report success in combining incompatible glass pieces with a larger, different base.

Questions arise.

Have the resulting pieces been tested for evidence of stress with polarised light filters?

Other destructive methods such as hot water, or placing in the freezer are not adequate measures of the long-term effects of incompatibility stress.  When you are doing something outside the accepted norms, then you must test for stress to be certain what you are producing remains sound before announcing success.

Why does glass with incompatible pieces survive?

Incompatible glass will show some stress when viewed through polarised filters. You will need to decide when it is excessive.  When viewed between polarised light filters high stress will be shown by a rainbow effect in the halo of light.  Lesser stress will be shown by pale light. The degree of stress will be shown by the amount of light.

Survivability

There are some circumstances where the glass can contain the stress, and others where it cannot.

Generally, large mass pieces can contain the stress from small incompatible pieces of glass. 

Spherical objects can contain a lot of stress over a long period, which is why glass blowers and lamp workers are generally less concerned about incompatibility than kilnformers are.

Flat glass pieces behave a little differently.

Circular forms can contain stress more easily than other shapes.  Rectangular  shapes generally show the most stress at the corners.  Narrow or wedge-shaped pieces have the most difficulty in containing stress.  The stress is concentrated at the points.

The placing of the incompatible glass is also important to the survivability of the glass.  The further from the edge of the piece, the less likely there will be breaks. 

The smaller the pieces of incompatible glass in relation to the whole, the less risk of breaking. 

The more spread apart the pieces are, the greater the chances of survival for a while or long term.

The most essential piece of equipment for people starting out and those who are investigating new setups or working at the edges of accepted norms is a pair of light polarising filters to test for stress.

When combining incompatible glasses the general case is that the greater the mass of the whole object in relation to the incompatible glass, the greater the chance of survival. 

Polarising Filters

Using polarized light filters to show stress works on the principle that stressed glass rotates the polarisation direction of the light as it comes through the glass. As polarized light filters placed at right angles do not allow any light through, only unstressed glass will continue to appear dark. 

If there is stress the light is rotated slightly and becomes visible through the filters.  

You can buy stress testing kits that incorporate a light source. You can also make your own. You need polarizing lighting gels. These come in sheets and are available from theatrical lighting sources. You will need to frame these in stiff card to keep them flat.

You use them over a light source. Place one filter down above the light source. Place the piece to be tested on top. Then orient the top filter so that the minimum amount of light shows through the filters. Any stress will show up as a light source.  The amount of light rotation depends on the stress direction, magnitude and light path length. The greater the intensity of the glow, the greater the stress the glass is exhibiting.   The amount light visible through the filters is wavelength dependent, as the filter transmits light with a particular polarisation direction. If there is large stress, different colours will be visible. 

This example shows extreme stress by the rainbow effect of light rotated in multiple directions

Note that the surface through which the light comes should be rigid, as any deformation of the surface will give a false reading.  The light filters through the slight curve and gives a stress reading, which may not be true at all.  Thus a firm flat surface is required, especially if you have a large light table for your light source.

Also note that the filters are normally on plastic sheets and easily scratched, so the glass should always be lifted and placed, rather than slid, to a new position.

A description of the compatibility test can be seen here.

revised June 2018