Hydrated Lime on the Market. A popular form of lime on the market is the hydrate. Manufacturers first burn the stone, and in the case of a pure limestone they drive off 44 pounds of each 100 pounds of the weight in burning. Then, they combine enough water with the lime to change it to hydrate form, and that adds 18 pounds weight. It is run through a sieve to remove any coarse material, and then packed in bags which help to exclude the air. The small packages in which it comes upon the market make handling easy, and this helps to bring it into demand. Its good physical condition makes even distribution possible, and thus permits maximum effectiveness to be obtained. It is only slaked lime, identical in composition and value with lime of the same purity slaked on the farm, but some dealers have been able to create the impression that it has some added quality and peculiar power. This does no credit to the public intelligence, but the hunger of soils for lime is so great that investment at a price wholly out of proportion to the price of farm-slaked lime has rarely failed to yield some profit.
Degree of Purity. It is always a reasonable assumption that hydrated lime has been made from stone of a good degree of purity. A local stone, burned on the farm, may be of low grade, but no man of business judgment would erect a costly plant for burning and hydrating lime where the purity of the stone would not afford a good advertisement in itself.
On the other hand, we find very little hydrated lime on the market that has not had sufficient exposure to the air to become changed in some part to an air-slaked condition, or has had refuse mixed with it. Air-slaked lime is not worth as much per ton as the hydrate because it cannot correct as much soil acidity, and the percentage of the former cannot be determined by the buyer. Its presence may not be due to any wrong-doing of the manufacturer, and, on the other hand, the increase in weight that attends air-slaking may be welcomed in some degree by a dishonest manufacturer before the goods are shipped. The difficulty in preventing hydrated lime from adding to its weight by becoming air-slaked is a point to be taken into consideration.
The percentages of air-slaked material in hydrated limes are widely variable, and no manufacturer can standardize his product on the market surely for the benefit of the farmer. In some instances the product is adulterated with refuse material in finely pulverized condition.
CHAPTER XIII
OTHER FORMS OF LIME
Air-Slaked Lime. A pure limestone is a carbonate, and the chemical formula is CaCO3. When it is burned, the carbon dioxide (CO2) is driven off, leaving CaO, which is calcium oxide, called fresh burned lime. In this process 44 pounds of a stone weighing 100 pounds passes into the air, and there remain 56 pounds of lime. When it air-slakes, it takes back the carbon dioxide from the air, and the new product becomes CaCO3, or carbonate of lime, and regains its original weight of 100 pounds. This is what would happen if the process were complete, and it is nearly so when the exposure to the air is as perfect as possible.
Fifty-six pounds of valuable material are in the 100 pounds of air-slaked lime, just as is the case with limestone, and there is no difference in effectiveness except in so far as the air-slaked material is absolutely fine and available, while most pulverized limestone is less so. In making purchase for use of land the buyer cannot afford to make any appreciable difference in price in favor of air-slaked lime, as compared with a fine stone.
Air-Slaking a Slow Process. Lime changes to an air-slaked condition slowly unless it has full exposure. Old heaps will remain in hydrate form for many years, excepting the outside coat, which excludes the air. Complete air-slaking would not reduce ability to correct soil acidity, the total amount of calcium and magnesium remaining constant, but weight would be added in the slaking, and therefore the value per ton would be reduced. The slowness with which air-slaking proceeds gives reason to expect that any bulk of old lime may contain a considerable percentage of the hydrate, and therefore have greater strength than a true carbonate like limestone. This is a consideration of value to a buyer.