Frost-resistance of a Pot
A bonsai pot is usually regarded as one unit, but glazed pots are composed of two different materials, which are the clay body of the pot and the glaze. The glaze, glossy or matt, ist the coating or the dress of a pot. Both the body and the glaze must match in their dilation characteristics, otherwise glaze fissures, glaze drips, blasted edges and similar glaze flaws can occur.
Let us regard the pot's body, the clay.
All sorts of clay are different because clay is a product of nature. You will hardly find two clay pits with the same clay quality. The potter must exactly determine which sort of clay he uses for his pots and at which temperatures he fires them to make them frost-resistant. The frost-resistance is calculated from the water absorptive capacity of the clay body.
Frost damage on a high quality Japanese pot.
Damage inside the pot.
(Photos: Dave Cochoy, Ohio USA)
1) The chips that have come off were caused by water soaked into the clay body of the pot.Then frost has blasted the chips out.
2) The crack in the corner of the pot was caused by the pressure of frozen soil against the pot's walls.
Take a small clay slab and weigh it after high-firing or take a ready fired pot. Then cook it for about five minutes, so that small air-filled cavities can fill with water. Then weigh the slab or pot again. The weight may not increase more than 2% if the pot is to be frost-resistant. (By the way, a potter must guarantee this!)
The tight-firing temperature is that at which the clay gets a water absorptive capacity of less than 2%.
The firing curve of a clay shows little contraction at the beginning (line 1). Approximately at 1000°C a contraction of the clay starts (line B1) up to a certain point at which the clay inflates and blisters for example (line C). From then on the clay melts together (line D).
The tight-firing curve lies before the bend of the curve (point A2). The bend marks the greatest contraction. The pyrometric cone equivalent (B2, for potters only) after the curve bend marks the greatest inflation.
Some clay almost tight-fires at 1200°C already but can endure a firing temperature up to 1700°C. It makes no sense however to fire bonsai pots this high just to reach a water absorptive capacity of 0.0% and it would be too risky too.
Up to 2% weight gain we speak of sintered ware, at 2 - 5% of semi-sintered (not frost-restistant), at more than 5% weight gain it is porose ware (absolutely not frost-resistant). So frost-resistance is linked with the porosity of a pot.
Porosity of a pot and the soil inside it
An important factor for the frost-resistance of a pot is also the soil. Normally the soil, especially Akadama-substrate, is porose enough to compensate the expansion of the ice inside the pot. A big drainage hole also helps to prevent the pot from filling up with water. There remain enough open pores in which the ice can expand without blasting the pot. (By the way, such damage must be covered by the owner of the pot).
The characteristics of the pot concerning frost (as described above) are connected to its porosity. The pot's body must not soak up water. By means of higher firing, the potter reduces the saturation ability. The pot must posess more closed and impermeable pores.
The pot's dress, the glaze
Porose fired pots must never be glazed because this would clog up the surface pores. Ice crystals then would be unable to emerge from the pores and would lift the glaze in chunks off the pot. A glaze alone does not make a pot frost-resistant. A tight-fired pot however does not soak up water, the ice can not endanger the pot so that it is frost-resistant. The pot and the glaze are a product of the fire. A bonsai potter must deal with the power of fire, because here most of the damages like firing fissures or glaze fissures occur. Glaze fissures, delicate to coarse ones, occur directly or some time after the firing process. These fissures are caused mainly by tensile stress of the glaze. Dilation differences between glaze and pot are responsible for this. This glaze flaw means a reduced frost-resistance!
Beware: Even craquelé glazes (in old China “cracked glaze”) are actually glaze flaws, only this flaw is produced intentionally. After high-firing delicate little tension fissures spread like cobwebs over the pot. Afterwards they are dyed dark to make them more visible. These pots also are not frost-resistant!
Two different firing processes, for example high-firing a pot, then applying a low-firing glaze and firing again, do not make a pot frost-resistant.
Another little frost-resistance test.
Hold the pot with thumb and forefinger (you can do this at your trader's or potter's place). The higher the sound when you strike the pot with the knuckles of the other hand, the more frost-resistant is the pot. The sound must be pure and long-lasting. If a pot is not frost-resistant, it has a dull sound and it does not last. Very experienced pot enthusiasts can even sense the frost-resistance of a pot with their tongue. Not frost-resistant pots feel as if you touched a blotting paper with the tongue. Frost-resistant pots feel like licking at a glass pane.
The “spittle test” works like this: put a small amount of spittle on your fingertip and then dampen the pot with it (on the inside of glazed pots). If the moisture is quickly absorbed, the pot is not frost-resistant; if the moisture stays long on the surface, it is frost-resistant.
The “Endurance Test”
All photographs and drawing by Peter Krebs
Translation: Heike van Gunst