These planters are heavily burnished in the making and then biscuit-fired. (To burnish is to rub the surface smooth with a spoon or pebble. It is an ancient finishing technique used in many cultures where glaze was not used or not yet known.) After biscuit-firing, these planters are placed in a garden incinerator full of sawdust that burns down overnight, leaving random but controlled markings of black on the pots. The blackening, which is carbon – like soot – results from the oxygen-reduced atmosphere around the pot and is darkest where the sawdust is most tightly packed. Several factors influence the variation in the blackening effects. Sometimes the affected areas look metallic, other times like ebony wood.
The planters are designed mainly to be used outdoors on garden walls and the like, so I guess the obvious question is, will they crack or spall if left outdoors in winter? I’ve been making them and leaving them in my garden (a typical suburban garden in Essex) for years and they have so far survived winter frosts without spalling or other damage. I do not know why. They are not ‘frost-proof’ in the normal sense: ‘frost-proof’ garden pottery is generally stoneware (fired to approx 1260C), so it is frost-proof by virtue of being utterly impervious to water, vitreous in other words. But that’s not very good as plant pot material: it’s the porous nature of low-fired pottery like terracotta that makes it so good for moderating the moisture content of the earth around the plant.
My planters are definitely porous. They are only fired to biscuit (1000C) and then to bonfire temperature (approx 700C) or less in the sawdust. I guess it is the open clay body (I use terracotta or crank clay) and the low temp of the firing that makes them withstand winter frosts without mishap. Or maybe it is just that my garden is more sheltered than most: I also leave terracotta flowerpots out over the winter and they generally survive well too.
Still, to be on the safe side, if I were to put my planters on sale, I couldn’t guarantee they’d be frost-proof and I would recommend that they are not to be left outdoors in frosty weather.