In past articles I have discussed how freeze/thaw has an effect on ceramic containers and I have also discussed clay pot irrigation. We are in the worst drought in the state of California’s history, so by irrigating with low fire pots with high porosity, water can be sent directly to the root system and feed plants on demand, as opposed to spraying the crusty, evaporative surface.
Technically porosity is the amount of empty space in the structure of the fired clay that makes it capable of absorbing or distributing liquids. Simple, right? So if the terracotta is absorbing water and the temperature drops below freezing, you’re going to have potential problems with your investment as the water expands while it freezes. But if your intention is to use the porosity to slowly distribute water, then the more porous the better. There are other uses for porosity in ceramics, for example insulation. The less dense a brick is, the more insulating capabilities it has.
Natural clays can have a very high porosity. When the grains are very small, there are countless small pore spaces, but what clay does NOT have is good permeability, which is the way you would measure how connected these porous spaces are. So generally speaking, if the clay is high fired it will be far less porous. This is advantageous for a freeze/thaw condition. If the clay is low fired, or under fired it will be porous and best for releasing or absorbing water.
So the bottom line is that the level of porosity you should seek depends on your intention for use of the ceramics.