These are melt tests of different types of porcelain stone from sometime at or before the Qing dynasty. Iron or blue qinghua underglaze was used to write place names and other notes on each test.
Porcelain stone is an amazing material that primarily comes from a number of mines near Jingdezhen. Combining porcelain stone with kaolin results in beautiful high-fire porcelain bodies, while combining porcelain stone with a flux like “glaze ash” or whiting gives a beautiful celadon glaze in reduction.
For more information, see one of my favorite books on Chinese glazes (and ceramics in general): Nigel Wood’s “Chinese Glazes”.