There is allways a problem with setting the annealing temperature right:
- Too high temperature will cause sagging of the glass.
- Too low temperature will not release all stress in the given time.
- Every kiln thermometer has a given offset or misreading.


You need:

- clear glass rods
- torch
- kiln with thermostat
- polarizer filter (linear)
- source of polarized light (LCD-screen or 2. polarizer filter)


Preparation of the glass and observation of stress:

The easiest way to introduce stress into a glass rod is melting one end and cooling it in the air. The prepared end should be free from visible cracks and
scum (bubbles, reduction). To make the stress visible hold the rod in front of the polarized light and look through the filter. Rotate the filter between rod and eye into a position that blocks most of the light from the source (dark field). A glass rod without stress should have a homogenous brightness. Stress shows up as brighter lines and clouds in the glass.


Science behind it:

Stressed glass turns the plane of polarization while the light passes the glass. The angle (and direction) of this rotation is proportional to the thickness and the amount and kind of stress in the glass. Pressurized glass rotates the light into a different direction than glass with tensile strain.


Testing the kiln setting for the strain point:

Put several prepared rods into different places in your kiln and run the usual (batch-) annealing programm. Analyze the stress in each rod as discribed abouve and take your notes. Prepare the rods again by melting the ends fresh. Set the kiln 10...30 degrees (C or F) lower and anneal the rods again. Once you detect remaining stress in the glass repeat the last, higher, setting to confirm propper annealing results.



Testing the maximum usefull temperature for your glass:

The kiln is too hot if the glass sags or bends under it's own weight. To test this put a long rod (30cm or 12") of the softest glass color you have (gold ruby, ivory or white Effetre) on two short supports (rods, kiln furniture, ...) and start the annealing cycle. Once the annealing temperature is reached look if the rod is still straight. Wait 10 (20, 30) minutes and control the rod again. If it is still straight increase the temperature by 10C (20F) and repeat the soaking process. Once the rod starts to bend you know the kiln was too hot at that time.


Adjusting temperature and time for your work:

Smaller workpieces don't sag so fast and they can be annealed at higher temperatures and shorter times. Bigger workpieces need longer soaking times and in tendency lower maximum temperatures. The ramps have to be adjusted to the maximum thickness of the material.



Final words:

The best annealing can NOT eliminate stress from incompatibility. It can not heal cracks from neglected reheating while making the bead. It can strike colors and opacifiers in the glass. The kiln atmosphere can cause or eliminate surface films as reduction or luster.