Ashes, the solid remains after the burning of combustible substances. When a vegetable or animal substance is burned with free access of air, part of it is resolved into volatile compounds, chiefly water, carbonic acid, and free nitrogen, while the other and generally the smaller portion is left as incombustible residue or ash. If the substance be decomposed with exclusion of the air, a different set of compounds results; and the residue may be charcoal, bone black, or some other substance, depending upon the nature of the material taken for the experiment. Of wood ashes, even the different parts of the same plant furnish different quantities, and ashes of different compositions. The soil itself has an influence upon the kind and amount of materials taken up by the plants. Nearly all the substances found in the soil enter into the composition of vegetable matters, and are found in their ashes. Alumina is, however, very rarely met with. No inorganic substances found in the ashes of plants come from any other source but the soil. Of the portion of wood ashes soluble in water, and removed from them by leaching or lixiviation, the greater part consists of the carbonate, silicate, sulphate, and chloride of potassium.
Of the insoluble portion (leached ashes), carbonate of lime commonly forms about one half; the remainder is mostly silicate and phosphate of lime, oxide of iron, and salts of magnesia. It is not supposed that the bases were combined with carbonic acid in the plants, but with organic acids, and that these were replaced by carbonic acid in the process of combustion. Plants that grow in and near salt water contain soda instead of potassa, deriving it from sea salt. The following examples show how the quantity of ashes varies with the wood: From 1,000 parts by weight of oak, well dried, Kirwan obtained of ashes 13.5 parts; from elm, 23.5; willow, 28; poplar, 12.2; ash, 5.8; pine, 3.4. The bark furnishes more ashes than the solid wood, and the branches than the trunk. Peat and coal ashes contain a large proportion of alumina; oxide of iron, carbonate and sulphate of lime, are also found in them. The principal uses of wood ashes are for making soaps and for enriching land. The soluble salts of potash are dissolved out from them, and oil or fatty matters added to the alkali, to produce the soap. The residue is a valuable manure, but evidently inferior to the ashes before the potash was extracted. Pot and pearl ashes are the salts of potash extracted from wood ashes.
The name potash is traced to the method of its preparation from the extract of the ashes boiled down in iron pots. Barilla, or soda ash, is a similar product of sea plants, soda replacing the potash. It was formerly largely imported into this country, but is now excluded by cheaper preparations of soda direct from sea salt. Ashes are sometimes used with lime and sand to increase the strength of mortar, and prevent its cracking. - Bone ashes contain much phosphate of lime, the cause of the fertilizing properties of bones. Phosphoric acid and phosphorus are prepared from these ashes. They are also used to make the cupels in which argentiferous lead is melted and oxidized for obtaining the pure silver. The cupels are merely bone ashes made into a paste with water, or beer and water, and then moulded and dried. - In distilleries, ashes find an extensive use for the rectification of the alcoholic liquors, the alkaline matters neutralizing any acids that may be present, and thus preventing their volatilization.
It is a common impression that their great consumption in American distilleries is to give strength to the liquors after their dilution with water, and this is confirmed by the violent caustic quality, not unlike that of the ley of ashes, for which much of the common whiskey of the country is remarkable. Ashes mixed with salt make a strong cement for iron pipes. Cracked pipes repaired with it bear as heavy pressure as new pipes. The cement sets on application of heat of 600°. - Shower of Ashes, a phenomenon which frequently accompanies the eruption of a volcano. Quantities of matter resembling fine gray or black ashes are thrown aloft from the crater to prodigious heights, and borne by the winds to an astonishing distance. On the eruption of the volcano Tomboro, in the island of Sumbawa, east of Java, in the year 1815, a shower of ashes fell for 19 hours in succession. An English cruiser, 100 m. away from the island, was surrounded by the cloud, and received from it an addition to its freight of several tons' weight, and a Malayan ship was covered 3 feet deep.
The ashes fell upon the islands of Amboyna and Banda, the latter 800 m. to the eastward, and this apparently in the face of the S. E. monsoon, which was then blowing, but really carried by a counter current, the existence of which in the higher regions of the atmosphere was then first established. A similar phenomenon was observed in the eruption, in January, 1835, of the volcano Coseguina, on the S. side of the gulf of Fonseca in Guatemala. Its ashes were carried to the eastward, over the current of the trade winds, and fell at Truxillo, on the shores of the gulf of Mexico. Ashes from Etna were deposited in Malta in 1329; and in A. D. 79 the cities of Herculaneum and Pompeii, which had 16 years before been visited by an earthquake, were buried beneath the showers which fell from the neighboring volcano of Vesuvius. Volcanic ash is a mechanical mixture of minerals and rocks absorbed by trituration against each other, and consequently exhibits great difference of structure and composition. Not being a product of combustion, it can hardly be called a true ash.