Any hydrophilic material that increases the viscosity of a liquid. Thickening agents are used in adhesives and paints. The main types of thickeners are (based on Lewis 1993): 1. Natural products: starch, gum, casein, gelatin, agar, etc.; 2. Synthetic cellulose derivatives: carboxymethyl cellulose, etc.; 3. Polymers: polyvinyl alcohol; polyacrylate; 4. Inorganics: clay, bentonite, silicates, colloidal silica. Synonyms: thickener; rheology modifier. Museum of Fine Arts, Boston (2000). [J. Cassar, R. de Angelis]