After reading a thread where crazing was referenced, I got interested in looking into it. Since starting Pour Horse in 1995, I've learned a lot about ceramics, and continue to read and research, even beyond the field of earthenware. I may be wrong on some points here, and if do please do drop me a pm and enlighten me
None of the horses in my case, dating back to the first Saucys, have crazed. With the exception of one, who crazed soon after firing, from a pour hole plug that was too dense. Other than that, every horse in my case is stable (pardon the pun) and exhibits no signs of crazing. Our weather is moderate, our humidity generally high but it rarely gets hot or cold here. So they have had very little variation in their environment.
However, HRs are known to craze rather badly. Royal Worcesters don't craze at all. Why the differences? If you grab a cuppa tea and hang out for a few minutes, we can go through the basics and it might help you to understand the differences between the horses and why some might craze and some will not.
First off, the very basic difference between earthenware (low fire clay) and porcelain/bone china/stone ware (high fire clay) is that the earthenware clay, when it is fired properly, still remains porous. If you had a bisque white earthenware horse, and spilled grape juice on it, the grape juice would stain the clay. It seeps into the little pores and could actually go all the way through. If you filled your earthenware bisque horse with water, like a flower vase, the water would sweat out of it, and evaporate, just like the sweat on your skin. Earthenware can't hold water, nor is it waterproof, even when it is fired properly. That's important to understand... even when the clay is fired as high as it is meant to go, it will still be porous.
In porcelain, bone china, and stoneware, the clay is fired much hotter... but more than that, each of these clays has something else in it, something that makes crystals grow inside of the pores so that when the horse is fired properly, there are no pores for water to get into. Your accidental grape juice would wipe right off. It wouldn't stain. That's why they use porcelain or stoneware for toilets, sinks, and old sewer pipes... because even without glaze, the water cannot go through them. The technical term is 'vitreous'. It is something already in the clay, a component that actually changes when the clay is fired and makes this happen. It won't happen with earthenware because earthenware doesn't have that ingredient. See, they really are different!
Glaze, the clear coating that is fired on the outside of the piece, is actually melted glass (plus other stuff to make it flow) When glaze is fired properly, it makes the earthenware horse waterproof ** on the outside**. But the horse is still not protected inside, where there is no glaze. The earthenware clay on the inside can still absorb water, and will do so... slowly... if the humidity is high. Or it might shrink when it gets cold, and expand when it gets hot. Humidity plus heat makes the clay expand like a hot wet sponge. Cold will make it shrink. Cold plus humidity inside of the clay will make the clay brittle over time, if the horse freezes and thaws. Cold is the enemy of ceramic horses... as well as ceramic flower pots, garden statuary, etc. Wet pots crack in a hard frost. Most horses won't be exposed to frost, but if stored in a cold garage, shed, or attic, the extremes of weather make them much more vulnerable. That's why so many Hagen-Renakers are crazed... they were likely stored for some period of time in unheated buildings.