For many years geologists searched for a substance which could be molded into any size and form, and would have the hardness of rock. As a matter of fact it was found that limestone was composed of carbonic-acid gas, clay, and lime, and that when great heat was applied the sealing bond was disrupted and the rock was reduced to a powder. When this powder was placed in water the gas was set free so fast that it made the water boil. The powder, or calcined rock, is now known as lime. This action demonstrated that nature used heat and moisture in forming these materials into rock. Knowing that clay contained silica, and that silica furnished the sealing quality of rocks, experiments were made to reverse the order of this rock formation, and a cement was produced. Equal portions of lime and clay were mixed together and stirred until all parts were thoroughly mingled, and then the mixture was subjected to a very high heat, after which the resulting mass was ground to a powder. When this powder was mixed with water, instead of the gases passing off as they did in the case of the lime, they penetrated the clay and the mixture became hard. This was first called Portland cement, as it was made from Portland limestone.
The Furnace is Built Up of Ordinary Brick and Used for Calcining the Lime
This discovery partly solved the problem of artificial-rock making, but not wholly, for the best makes will break, peel, and crack without the slightest cause and when least expected, and besides its dark-gray color and rough appearance is unattractive. Much progress has been made with cement for interior decorations and many of the finest marbles are closely imitated. This grade of cement will not weather and its use is confined wholly to interior work. A white cement is much desired and many of the large manufacturers maintain laboratories where experiments are carried on constantly in the endeavor to produce it.
To build a miniature cement plant, first secure sufficient common brick to make a furnace with an inside cavity, 20 in. square and 24 in. high. Two sides and one back wall are built up, sealing the brick with mortar, clay, or cement. The bottom is covered with bricks standing on edge, and so placed that they will be about ¹⁄₄ in. apart, to serve as a grate. The top is then covered with a piece of tin, or asbestos, and a hole is cut in its center to receive a pipe, about 3 in. in diameter, for a chimney. This chimney should be about 15 in. high. Build the front wall halfway up by laying the brick loosely together so that the fire will get the air through the crevices, then cover the grate with kindling, place coal on top of it, and start the fire. When it is well under way, place a few fair-sized lumps of limestone on top of the fire and complete the wall to the top by laying the brick as closely as possible. Use sufficient coal to burn at least two hours.
When the fire has burned itself out and the furnace has cooled, remove the front wall and take out the burned limestone. Some parts of the limestone will be mixed with the coal, but most of it will remain in the lump, which is known as “black lime,” and when it is placed in water it will give off gas very fast. Obtain some fire clay and thoroughly mix equal portions of lime and clay, then place the mixture in a one-piece pan, made of pressed tin or sheet iron, as a soldered-bottom pan will come apart with the heat. Build up the front of the furnace as before, build a fire and place the pan on the fire and let it burn itself out. When the furnace has cooled, remove the front, take out the pan, and pulverize the mass in the pan. When this powder is placed in water it will become hard. If some sand or gravel is mixed with it, and the mortar thus formed is spread out over a flat surface, a miniature cement sidewalk will be the result.
Gauge for Laying Out Model Gear Wheels
The Gauge Steps Off Each Tooth Accurately for the Saw to Cut the Next Slot
The illustration shows a simple device for making small gear wheels by hand. It is made of a piece of brass, ¹⁄₈ in. thick and about 5 in. long, shaped as shown, forming a tooth, A, according to the size of the teeth required in the gear wheel, with the end B on a radial line from the center of the wheel. Use one or more hacksaws, according to the size of the slots or teeth to be cut. If one blade is not large enough and two are too large, grind off the teeth on the inner side of the saws, or if two are not large enough, place a thin piece of paper or metal between them to make the thickness required. Make a clamp, C, for holding the blades together, by using a piece of sheet brass, 6 in. long and ¹⁄₂ in. wide. This clamp also acts as a depth gauge for the slots.