Tuesday, June 23, 2020

Crystalline Glazing. More hours of fun... using temperature and time to creatively shape our crystals.


Lark ascending...

Many years ago I remember watching a skylark ascend into a blue cloudless sky. Rapid beats of its tiny wings lifted it steadily higher and higher, all the time circling and singing. At last, it was so high that it was almost impossible to see. I dared not look away or blink my eyes lest I lost sight of it and could not locate it again. Then, still singing merrily, the lark folded its wings, dropped like a stone, abruptly halted its dive, hovered for a few seconds, then dropped again and hovered some more. Every time it hovered it was as though it had reached a solid platform in the air. Eventually it fell quickly to the earth and made a safe landing just out of sight over the curve of the hill top.

Curiously this memory of the lark’s happy climb to altitude followed by the pattern of rapid fall, plateau, then rapid fall reminds me of firing crystalline glazes!

Crystals arranged themselves like thistledown where the crystalline glaze overlapped the liner glaze on the inside of a large vase.

Previously....

In the previous blog post I wrote about 6 things that can determine how many crystals grow on a crystalline glazed pot. The first three were concerned with clay body and glaze, and the rest related to the firing itself.

Just like a happy lark, our kiln has ascended to peak temperature. By cleverly using our rate of climb, our maximum temperature, and how long we have lingered there, we have influenced how many crystals will remain for us to grow on to a large size later in the firing.


The Fall!

Now the kiln is switched off and the temperature allowed to fall until a zone is reached in which the crystals will grow well. If the peak of the firing has been around 1280 – 1300 Celsius (2336 - 2372 F) then we are aiming for a point somewhere around 1100 Celsius (2012 F).

Some exponents of crystalline glazing open all the vents of the kiln, or even crack the door or lid in order to speed the initial descent. Opening kiln lids at 1300 Celsius (2372 F) is potentially lethal and I do not do this, but I think it needs mentioning that some do! You may recall that crystalline glazes are abnormally fluid when at peak temperature, and will be sliding off the pot like molasses or treacle, the abrupt fall in temperature will prevent further glaze loss.


Getting in the Zone...

The zone in which crystals will grow is quite broad, somewhat greater than 150 Celsius (302 F), and the interesting thing about it is that the way the crystals arrange themselves will look quite different if they are grown at a hotter temperature, or at a cooler one. In simple terms, crystals will arrange themselves in rounded shapes if grown at the cooler end of the zone, but will become progressively more spiky looking towards the hot end of the zone.

Spiky


Somewhere in between the two extremes crystals tend to arrange themselves in axe shaped clusters.


The fun begins!

And that is just the start! A slow fall of temperature over several hours can give a star shaped centre to the crystals.

A star in the middle, surrounded by growth rings.

A dark ring, almost like a cartoon outline, can be put around crystals by dropping to about 1000 Celsius (1832 F) at the end of the main growing period, and holding for a further hour. This dark ring is particularly noticeable in glazes containing copper.

Cartoon Outline.


Growth rings can be put in crystals by abruptly lowering the temperature by 40 – 60 Celsius (104 - 140 F) from the growing temperature, holding for a few minutes, then quickly returning to holding temperature. Pale growth rings can be made by rapidly raising the temperature, holding for a few minutes, then dropping back to the holding temperature.

Growth Rings.


Putting close spaced growth rings in crystals is hard work when you have a manual kiln. The frequent temperature changes mean much adjusting of the simmerstat switches, but it can be done! If a kiln can gain or lose heat quickly growth rings can be closely spaced. If the kiln is slow, then rings have to be far apart, that is one good reason that crystalline glaze firings are best done with only a few pots at a time and little if any kiln furniture.

Crystals with dark centres and pale outer growth can be made by starting growing them at a temperature that is at the low end of the zone, say about 1050 Celsius (1922 F), staying there for two or three hours, then rapidly climbing to around 1130 or 1140 Celsius (2066 -2084 F) and holding for two or three hours more.

These crystals had 2 hours growing at low temperature followed by 3 hours at a much higher temperature. The high temperature gives a spiky outer fringe.


These were grown with the timing reversed, 3 hours for the middles and 2 hours at higher temperature for the outer fringe.

The hours in which the crystals grow is the most creative part of crystalline glaze firing. How long the kiln is held will determine size, and the temperature will influence the way the crystals look.

As you can imagine, there are numerous variables that feed into what the crystals finally end up looking like, but I hope this post, and the one before it, give some idea of the process of firing crystalline glazes.


Homework!
It would be very useful to explore crystal growing temperatures over a series of firings. Start with a firing that has a crystal growing hold at 1100 Celsius (2012 F). Follow this with a series of firings that step the hold temperature upwards. Then try some that hold below 1100 Celsius (2012 F). Compare the results.

Notes

Firing times are long, and potters can become tired and forgetful! It helps to plan what you want to achieve before the firing. Sometimes I will sketch a crystal on paper, and think about how I would fire the kiln to achieve its size and shape. If there are to be growth rings fired into the crystals, I need to think about the timing of when to change the temperature of the kiln to achieve them.

The thread-like rods of the individual crystals are visible in this close up.

I should note somewhere, and here will do as well as anywhere else, that zinc silicate crystals are rod shaped, and what we usually refer to as “crystals” are actually structures that are made up of numerous rod shaped crystals that cluster together.