The production of cast phenolic resins requires pure materials, expensive equipment, and extreme care in the control of the operation. A mixture of phenol and formaldehyde and a catalyst (usually sodium or potassium hydroxide) is charged into a nickel-lined reaction kettle and heated until the water separates and is removed. The reaction is then allowed to proceed to the desired point. Glycerin is added to aid in forming a transparent product. All equipment, including pipe lines, valves, and pumps, is nickel or nickel lined except that used for formaldehyde, which is made of aluminum. The resin is usually made in 1,000 pound batches, and the reaction cycle ranges from 6 to 18 hours. It is colored with soluble coal-tar dyes and cast into lead molds. These are placed in a heated room and allowed to cure for 3 to 6 days. The resin is removed from the mold with air hammers, and the lead molds are melted. The appearance of the resin may be changed by varying its water content, by the addition of dyes and fillers, and by the addition of other substances to produce some desired effect, such as imitation ivory or marble. The clarity of the resin depends upon its water content—the greater the degree of dehydration the clearer the product. Range of colors is complete, from crystal clear to the darker shades, with any degree of transparency, translucency, or opaqueness. Casting is in the form of sheets, rods, tubes, or special forms suitable for the production of buckles, jewelry, and other small products. Molds of complicated shape cannot be used, which means that most articles if produced of cast resin must be produced from standard shapes by subsequent working. Recently small radio cabinets have been cast. Uses. Cast phenolic resin can be machined in the same manner as hard wood. It must be polished after machining, usually by tumbling with shoe pegs and pumice or with muslin wheels. The smooth finish and low degree of heat conduction give the material a pleasant feel, not cold to the touch as is metal. The coloring is not superficial and therefore does not chip or wear off. Electrical properties are excellent. A slow polymerization continues for some time after fabrication, resulting in slight shrinkage. Cast phenolic resins are marketed by the producers as rods, sheets, cylinders, and special castings. Standard round rods range from ⅜ inch to more than 5 inches in diameter. Special rods are available in such forms as square, hexagon, octagon, and fluted. Standard sheets are in sizes from 12 by 24 inches to 36 by 72 inches, and from ⅛ to 1 inch thick. Stock cylinders are available in a wide range of inside and outside diameters. [Illustration: CAST PHENOLIC RESINS, STANDARD SHAPES AND SMALL ARTICLES FABRICATED FROM THEM. Source: Bakelite Corporation, 247 Park Avenue, New York, N. Y.]