Wiktionary
a. (context pottery English) Describing the condition of a clay or clay body when it has been partially dried to the point where all shrinkage has been completed.
Wikipedia
In pottery, leather-hard is the condition of a clay or clay body when it has been partially dried to the point where all shrinkage has been completed, and it has a consistency similar to leather of the same thickness as the clay. At this stage the clay object has approximately 15% moisture content. The clay is still visibly damp (normally a darkish grey, if it began whiteish) but has dried enough to be able to be handled without deformation. The body is able to be gouged or incised without breaking.
At this stage details such as handles and spouts are often attached to the main body, and many forms of decoration, including painting, may be applied.
Usage examples of "leather-hard".
Still, he pulled a piece of jerky from his saddlebags and let the leather-hard meat soften in his mouth before chewing.