Bath foam preparations may be in the form of liquid (or gel) with water, or as solids in the form of powders, grains, or tablets. Liquids must be formulated to retain uniformity as solution or otherwise on storage at expected temperatures, and preserved against microbial growth and oxidative breakdown. Powders may contain various solid diluent ingredients which in some cases will have liquid components adsorbed to them, and have their own challenges in terms of retaining uniformity against sifting of different densities of components. For various reasons, although powdered products were more common at first, liquids have come to dominate the market.