Preparation of zinc oxide
57 g of zinc sulfate are dissolved in 700 ml water and 23 g of sodium carbonate are dissolved in 500 ml of water. The sodium carbonate solution is the filtrate and poured into the zinc sulfate solution. Immediately the precipitate of basic zinc carbonate forms, which should be pure white. The solution is stirred to aid the escape of carbon dioxide and then the bottle is filled to the top with water. The suspension is let to stand for 30-60 minutes until the white precipitates are settled to one-third or one-fourth the volume of the bottle. The clear solution is decanted and the remaining precipitates are washed with distilled water by decantation. Finally, the sludge is transferred to a large, ordinary filter, and allowed to drain over night. The next day a jelly-like cake is removed from the funnel without removing it from the filter, unfolded without tearing, spread flat on paper towels, and dried. The obtained basic zinc carbonate is heated in the porcelain dish gently with a flame 5 centimeters high which is held in the hand and played over the bottom of the dish. The material falls to an impalpable powder which seethes, or appears to boil, as long as gases are being expelled. The heating of the dish to any faintest sign of visible redness should be avoided. Zinc oxide is faintly yellow when hot but it should become pure white when cold. The final product is kept in a stoppered bottle.
Synthetic inorganic chemistry, by A. A. Blanchard, 230-231, 1936