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| credit: Tia Murphy |
There are advocates for making tiles as references for future work.
- They show the profiles achieved at different temperatures.
- They can be stored for easy visual reference when planning a firing.
- It is a useful practice for any kiln new to the user.
These tiles are assembled in identical ways to enable comparisons. They should include black and white, iridised pieces- up and down, transparent and opal, and optionally stringers, confetti, millefiori, frit and enamels.
The tiles are fired at different top temperatures with the same heat up schedule with the top temperature of each at about 10C or 20F intervals. These show what effect different temperatures give. Start the temperature intervals at about 720C or 1330F.
This is a good practice, even if time consuming. It gets you familiar with your kiln and its
operation. It gives a reference for the
profiles that are achieved with different temperatures at the rates used.
Ramp rate and time
But, as with many things in kilnforming, it is a little more
complicated. The effect you achieve is
affected by rate and time used as well as the temperature.
The firing rate is almost as important as the temperature.
- A slow rate to the same top temperature will give a different result than a fast rate.
- The amount of heat work put into the glass will affect the temperature required.
- Slow rates increase the time available for the glass to absorb the heat.
- Glass absorbs heat slowly, so the longer the time used by slower rates, the rounder the profile will be.
Since time is a significant factor in achieving a given
profile, any soaks/holds in the schedule will affect the profile at a set
temperature. A schedule without a bubble
squeeze will give a different result than one with a bubble squeeze at the same
temperature.
To help achieve knowledge
of the rate/time effect, make some further test tiles. Use different rates and soaks for the test
tiles of the same nature as the first temperature tests. But vary only one of
those factors at a time. Consider the results of these tests when writing the
schedule for more complex or thicker layups.
Mass
Also be aware that more mass takes longer to achieve the
same profile. Slower rates and longer
times will help to achieve the desired profile at a lower temperature. It is probably not practical to make a whole
series of test tiles for thicker items.
But, a sample or two of different thicknesses and mass will be helpful
to give a guide to the amount of adjustment required to achieve the desired outcome.
The results of sample tiles are due to more than just temperature. They are a combination of rate, time, and temperature (and sometimes mass). These factors need to be considered when devising or evaluating a schedule, because without considering those factors, it is not possible to accurately evaluate the relevance of a suggested top temperature.
See also: Low Temperature Kilnforming, available from Bullseye and Etsy
