There are a few things to think about when you choose your container for fermentation. The jars will host your ferment for at least three weeks so they need to be of good quality. Avoid using metal containers as the ferment can corrode the surface of the metal and this can react with the contents in a negative way. Also avoid using plastic, as the acidity of the ferment can leach out chemicals from the plastic even if it is food-grade or BPA-free. You may see some people using plastic to ferment in, and it is practical, but personally I would refrain from doing this in case it leaches chemicals on to your food that are unhealthy to ingest. The best materials to use are ceramic, clay or glass containers. Make sure they are all lead-free; some older crockpots might have lead in the glaze for instance. Also make sure your jar is free of cracks as these can introduce contamination.