Water-cooled impression trays are used with reversible hydrocolloid impression materials. These trays feature an interior lining where cold water can be pumped through in order to properly cool the impression material when capturing an impression. These trays are often fabricated from metals capable of effectively transferring the temperature of the cold water to the hydrocolloid material. if you are using reversible hydrocolloid materials, it is important to have a supply of water-cooled trays on hand.