Wood recommends that, for the puering of sheep-skins, dung should be allowed to ferment one month before use, and states that it deteriorates if kept over three months. The puering products are the result of the successive action of many sorts of bacteria, and Wood is of opinion that those actually concerned in puering originate from the air, or from the vessels in which the dung is stored, and are not present in it when excreted. Borgman[111] advises that the dung should be kept in a dry condition, and only made into a paste between a fortnight and three weeks before use, by covering in a clean cask with cold water, and on the following day mixing to a smooth paste with a clean wooden “poss-stick,” made from wood free from tannin. The cask should then be covered up, and allowed to rest undisturbed till required. Clean extract-casks are very suitable for the purpose, if carefully and repeatedly steamed out, and Borgman advises that a regular series should be arranged, so as to supply the dung required, the date of mixing being carefully marked on each cask. Throughout the process the utmost cleanliness should be observed, and the casks should be carefully steamed out as soon as emptied. Immediately before use the dung-paste should be heated by steaming nearly but not quite to boiling point, care being taken to avoid the introduction of condensed water containing iron, and the dung thoroughly mixed with a large quantity (say 100 gallons) of water at 45°-50° C., allowed to settle, and drawn off through a basket, and strained into the puering paddle through a second basket lined with coarse open canvas (such as is used by plasterers to cover windows while the plaster is drying). A further quantity of warm water should be poured on the residue in the mixing tub, and used for diluting that in the paddle to the proper volume. The temperature of the liquor may reach 42° C. before the skins are introduced. The liquor should be of a light colour, greenish to brownish yellow; if darker, it indicates decomposition of the dung by improper storing, or too long fermentation, and will be liable to cause staining and injury to the skins. About 33 liters of dry dung is required per 100 kilos. of wet skin prepared for puering (33 gallons per 1000 lb.). Dry dung should be of yellow to brown colour, dark brown or black dung is spoiled and unsuitable for use. Wet dung is more difficult to judge, but very dark brown or black should be rejected, as well as that with a very strong smell, indicating that it has already fermented. Borgman’s directions bear the stamp of experience and common sense, and the book as a whole repays study. [111] ‘Die Feinleder-Fabrikation,’ Berlin, 1901, p. 69.