BF slip cast forms are made from an ultra fine porcelain which is fired to vitreousness. This means that the clay basically becomes like a glass and the vessels are completely water tight. They are finished with a fired matt underglaze and then polished with a fine emery. To keep your BF forms looking their best they can be dusted with soft cloth or wiped down with warm soapy water. If they lose some of their luster they can be buffed with a furniture polish, preferably one that is bees wax based.