Pumice

“ I've seen, at other volcanoes, pieces of pumice blown up so high that they've been caught by the upper currents of the atmosphere and carried away in an opposite direction to the wind that was blowing below at the time. Ay, I believe that dust is sometimes blown miles up into the air. ”

“ The water always carried such quantities of large angular pumice fragments, not to speak of sand and mud, that it was out of the question to attempt to wash in the brooks. If we tried, the pumice would so grind into our flesh as to prohibit any further efforts as cleanliness. But while washing is a matter of choice, one must drink whether or no. We were obliged everywhere to strain our water through one of our foodbags. Often we would have to strain a quart of pumice to get a pint of water. ”

“ Those who sought to flee through the streets were clogged by the small, loose pumice stones, which lay many feet deep, or were entangled and overwhelmed in the mud-streams, or were struck down by the rocks which fell from the heavens. If they escaped these dangers, blinded by the drifting ashes and groping in the dark, not knowing which way to go, they were overcome by the sulphurous vapors, and sinking on the highway were soon buried beneath the volcanic matter. ”

“ Eight above our heads, at a great height, opened the crater of a volcano from which escaped, from one quarter of an hour to the other, with a very loud explosion, a lofty jet of flame mingled with pumice stone, cinders, and lava. I could feel the convulsions of nature in the mountain, which breathed like a huge whale, throwing up from time to time fire and air through its enormous vents. ”

“ To my mind, a deposit of a light substance like pumice, which ordinarily floats on the surface of the water, is only possible in enclosed sheets of water, which would not allow of its escape. If it once reached the open sea it would be carried far and wide by the winds and currents. There is one thing, however, which should not be forgotten, and that is, that the enclosed air, to which pumice owes its buoyancy, might under pressure be driven out, in which case, of course, it would become water-logged and sink, and would then form regular aqueous deposits like sand or clay. ”

“ Pumices are most abundant and most typically developed from acid rocks; for which reason they usually accompany obsidians, in fact in Lipari and elsewhere the base of a lava flow may be black obsidan while the upper portion is a snow white pumice.
Small crystals of various minerals occur in many pumices; the commonest are felspar, augite, hornblende and zircon. If they are abundant they greatly diminish the economic value of the rock, as they are hard and wear down more slowly than the glassy material; consequently they produce scratches. ”

“ The reason why they can float seems to be that the clay of which they are made is like pumice-stone. So it is light, and also it does not, after being hardened by exposure to the air, take up or absorb liquid. So these bricks, being of this light and porous quality, and admitting no moisture into their texture, must by the laws of nature float in water, like pumice, no matter what their weight may be. They have therefore great advantages; for they are not heavy to use in building and, once made, they are not spoiled by bad weather. ”

“ Besides this generally distributed basic pumice, the stones in the agglomerate of the necks likewise include fragments of older volcanic grits or tuffs, blocks of basalt or diabase, as well as pieces of the Carboniferous strata of the district, especially shale, sandstone and limestone. Not infrequently also, they comprise angular blocks of fossil wood.
The materials which fill the necks are generally much coarser than those that form intercalated beds. ”

“ To overcome these, the “Kupramite” canister was developed. The filling consists of pumice stone impregnated with copper sulfate. Pumice stone, 8 to 14 mesh, and technical copper sulfate are placed in an evaporating pan in the ratio of one part by weight CuSO4·5H2O to 1.5 parts pumice, and the [Pg 231] whole is covered with sufficient water to dissolve the salt at boiling temperature. The mixture is then boiled down with constant stirring until crystallization takes place on the pumice and the crystals are nearly dry. ”

“ The upper part of this layer of pumice is of a brilliant red; and when the sun is shining upon it, is as bright and vivid as vermilion. It is porous and open, and its specific gravity but trifling. These curious bluffs must be seen as they are in nature; or else in a painting, where their colours are faithfully given, or they lose their picturesque beauty, which consists in the variety of their vivid tints. The strata of clay are alternating from red to yellow—white—brown and dark blue ”

“ As on the surface of the glass pot is formed a frothy-like mass which cools as a light, spongy, vesicular material, so by the explosions from a trachytic volcano, similar masses are formed and thrown out, well known as the useful pumice-stone. This variety of lava produces often a very ragged surface, much less durable to mechanical agents than that of the other class. ”

“ The surface of a modern skin may be greatly improved by “pouncing” but there seems to be a danger of its becoming rough or porous.
Pounce.—Fine powdered pumice (as supplied by drysalters) is very good. It is rubbed on with the hand (p. 167) , or with a pad or a piece of rag. Law-stationers use a pounce in which the main constituents are chalk (or “whiting”) and powdered resin. The latter, when used before gilding, is apt to make the gold-leaf stick to the surrounding parchment. (Before Writing, see Note 7, p. 359.) ”

“ Lower initial concentrations are secured; this is due, in part of course, to the smaller volume of phosgene in the shell containing pumice. Pumice does seem to keep the booster from scattering the phosgene so high into the air, and at the same time does not prevent the phosgene from being liberated in a gaseous condition. This would indicate that pumice gives a more even and uniform dispersion and a more economical use of the gas actually used. ”

“ In all that time she has poured out millions of tons of matter—lava, huge glassy boulders, little pebbles of pumice stone, long shining hairs, fine dust or ashes. All these things are different forms of melted rock. Sometimes the steam blows the liquid into fine dust; sometimes it breaks it into little pieces and fills them with bubbles. At another time the steam is not so strong and only pushes the stuff out gently over the crater’s edge. ”

“ I do not pretend to say, that a just estimate can be formed of the great age of Volcanos from [Pg 99] this observation; but some sort of calculation might be made: for instance, should an explosion of pumice cover again the spot under which Pompeii is buried, the stratum of rich soil abovementioned would certainly lie between two beds of pumice; and if a like accident had happened a thousand years ago, the stratum of rich soil would as certainly have wanted much of its present thickness, as the rotting of vegetables, manure, &c. is ever increasing a cultivated soil. ”

“ In different cavities of the pumice bed, there are large deposits of black ashes. Towards the extremity of the strata the red stone appears on the surface in a more solid state, and lies in prismatic masses, the prisms being small, and not exceeding a few inches in diameter. Their substance is brittle and crumbles with ease. This stratum of red lava is of a short continuance. Passing a small brook, it dips rapidly to the westward, and in its place. ”

“ Frank remarked, as Fred paused. "What is obsidian?"
"It is a mineral substance," replied Doctor Bronson, to whom the question was referred, "and is formed by the cooling of the lava from a volcano. When lava cools it forms into obsidian and pumice. Everybody knows what pumice-stone is. Obsidian is a substance hard enough to scratch glass, and is capable of taking a high polish and a keen edge. ”

“ Then the pumice-covered surface should be pressed against the face of the felt or wood wheel, and as 39soon as the pumice is removed by the action of the wheel more wet pumice-mud should be added so that the enamel surface may constantly be kept covered with wet pumice.
It is best to use the finest ground pumice obtainable, although on rough jobs a coarse grade may be used. Tutty powder (oxide of zinc) is said to give even a better polish than fine pumice, and is often used after a preliminary polish has been given by pumice. A different wheel should be used for each polishing compound. ”

“ The facts now detailed clearly show that vast torrents of mud must have passed through the streets of Pompeii, since dry ashes and ejections of lapilli and pumice, unaided by water, could never have found their way into the interior of closed amphoræ, nor made perfect molds of the human form, nor left a level water-line upon the inner walls of close arched passages. ”

“ In a short time the fire increased to such a degree, that it burst open the earth in this place, and threw up so great a quantity of ashes and pumice-stones, mixed with water, as covered the whole country. ”

“ Etching, dry-point, the burnisher, and a tone produced by roughening the surface of the plate with pumice-stone or some similar material, all are called into play to produce a harmonious result, and of their kind there is nothing finer. ”

“ Volcanic Ash or Tuff, Trap Tuff.—Small angular fragments of the scoriæ and pumice, above-mentioned, and the dust of the same, produced by volcanic explosions, form the tuffs which abound in all regions of active volcanoes, where showers of these materials, together with small pieces of other rocks ejected from the crater, and more or less burnt, fall down upon the land or into the sea. Here they often become mingled with shells, and are stratified. Such tuffs are sometimes bound together by a calcareous cement, and form a stone susceptible of a beautiful polish. ”

“ The quality of the rubbing accomplished depends much upon the selection of the blocks of pumice stone. The blocks of light weight, open grain, tunneled with innumerable air cells, are to be preferred for good cutting properties. Immersed in water, they float instead of sinking. The buoyancy of a piece of lava determines its porosity and its cutting power. This kind of stone may be used until the surface is well reduced, when, preferably, the stone of closer texture and tighter grain may next be used and continued in use until the final dressing up has been concluded. ”

“ While emery is used for polishing tools, polishing sand for stones and glass, ferric oxide for fine glassware, and lime and felt for metals, pumice stone is more frequently employed for polishing softer objects. Natural pumice stone presents but little firmness, and the search has therefore been made to replace the natural product with an artificial one. An artificial stone has been produced by means of sandstone and clay, designed to be used for a variety of purposes. ”

“ According to this view, the differences between some of the most remarkable igneous rocks are merely in the degrees of arrangement to which their particles have been subjected. As lava, obsidian, and pumice, are merely three states of the same volcanic product, so probably the granitic, porphyritic, and homogeneous rocks, generated by heat in ancient times, have derived their characteristic structures from the conditions of their solidification. ”

“ The work, after it has been coated with copal varnish and allowed to stand untouched for a few days, is ready for flatting or felting down with finest pulverized pumice-stone, rubbed with felt and water. ”

“ If the surface is in good condition it will be sufficient to scrub down with good soap and water and afterwards sponge and wipe dry. If the work has become rough it will often be necessary to use pumice stone to facilitate the operation of cleaning. ”

“ The glassy lavas, when distended by escaping gases, give rise to the formation of pumice, the white colour of which, as in the case of the foam of a wave, is due to the reflection of a portion of the light in its frequent passage from one medium to another—in this case from air to glass, and from glass to air. The volcanic bombs formed from glassy lavas are often of especially beautiful and regular forms. Sometimes the passage of steam through a mass of molten glass produces large quantities of a material resembling spun glass. ”

“ Now, two of the most important things you will have to guard against is cutting through the rough stuff and lead coats, and scratching the surface. There is a great difference in pieces of pumice stone. Some are hard and full of flint like particles, which will scratch the work; others are softer and of more even grit; the light colored and fairly open grained pieces are the safest to use. You can tell a fast cutting stone by its open grain and lightness. The finer grades of German rubbing brick and English rubbing stone are also used in rubbing rough stuff. ”

“ Pit coal and jet are matters which also belong to clay, and are commonly under slate. By the word tuffa, I understood not only the common pumice which appears full of holes, and, as I may say, organized, but all the beds of stone made by the sediment of running waters, all the stalactites, [230] incrustations, and all kinds of stone that dissolve by fire. It is no ways doubtful that these matters are not modern, and that they every day grow. ”