This unprepossessing grey mess I collected in a bucket is boulder clay. If you’ve got a house with clay walls, it’s like finding a seam of valuable minerals. There’s no clay to beat it.
Our part of Suffolk mostly sits on thick layers of the pale grey, white-flecked natural building material, laid down by glaciers grinding their way over chalk. But it’s usually deep down and hard to find.
It comes near the surface here, because that’s why our pond exists, sitting on an impermeable bed of clay. But it’s not so easy to dig out without draining the pond, which we think was once a clay pit.
Digging a trench to plant a hedge yesterday I kept finding lumps of it, deposited there after earlier work on the pond. It has got muddy, but that doesn’t matter – a bit of earth won’t spoil its performance.
Not only does it make a good, plastic clay for wall repairs when mixed with straw, it is also less prone to shrinkage cracks than ordinary yellow clay, and dries as hard as a lump of chalk.
If you have clay daub walls and come across any, store it till you need it. Wonderful stuff.
saving The Old Brewhouse 
So interesting. This is something that would not have occurred to me to look out for. I guess you have to have that interested eye that comes from living in a house built with these sorts of materials.
Hidden gold of the clay kind!