The most suitable clay to use is red terracotta clay, collected from a site free of weed seed. Often, digging deep will ensure that no unwanted seeds become part of the mix! The clay needs to be dried and ground finely to ensure a lump free mixture. Two bricks can serve as an effective grinder. Other clays, the blues and whites are not suitable for this process, as they contain minerals, which interfere with the growth of the seedlings.