Page:Popular Science Monthly Volume 91.djvu/787

 Popular Science Monthly

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��Simple Die Casting for the Home Shop

A SIMPLE and easily made mold for the use of the home mechanic is shown in the illustration. It consists of a length of tubing of any convenient material, cut longitudinally into two equal parts. Steel caps fit close over both ends of the two parts of the tubes, holding them firmly to- gether and in line. In use, one-half of the shell is first ,..

filled with plas- ter of Paris, a block of wood being used at each end to re- tain the plas- ter. The pat- tern, having been oiled with lard, or with graphite-filled oil, is embedded halfway in the plaster. After the plaster is set, the surface of the first half of the shell is carefully wiped over with graphite, and then the second half is filled in the same way. The parts thus placed to- gether are all capped and put away to set.

When the plaster has set and become dry the caps are removed, the tube taken apart, and the pattern removed. The core A is now set and the pouring gate B and riser C are cut in the plaster. The parts are then put together and the caps re- placed. The mold is then ready.

The great advantage of this mold is that its alinement is positive and automatic. This mold is intended for white metal, pewter and metals fusing at low tempera- tures. The drawing shows how the gates and risers should be located with reference to the pattern in all castings. If the mold is well dusted with graphite before each cast, and a proper number of cores supplied, a number of castings can be made from it.

This mold is especially adapted for mak- ing metal patterns to take the place of wood. — J. B. Murphy.

��Painting on Cemented or Concrete Surfaces

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��WHEN paint is applied directly cemented surfaces the free lime

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the cement will saponify the oil of the paint and destroy its integrity so that finally the paint will peel off. After such a surface has been exposed to the elements for about a year, paint may be applied with little danger of deterioration. But I this is not sure.

both ends to^?= :::= p^^ tit caps

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��Mold made in plaster to shape metal castings

��Expansion Shield for Screws Made of Sheet Lead

AVERY good expansion shield is made from sheet lead rolled up to such a size as to snugly fit the hole.

��One of the early processes for neutralizing the free lime was by the use of dilute muri- atic acid of 7 or 8 per cent of concentration mixed with water. But it was soon found that the acid used to neutral- ize the lime converted it into calcium chloride, injuring the cement surface so that it crumbled or pitted badly. An- other and better method of pre- paring the surface consists in the application of carbonate of am- monia, io lb. to 45 gal. of water. Insoluble calcium carbonate was formed and a large amount of ammonia was liberated. This preparation does not injure the surface of the cement. When used on a mortar plas- ter containing lime, as in building a stucco house, it is better to apply two weak solutions of this kind, rather than one strong solution.

But the best treatment is with zinc sulphate, used with water of equal weight. This wash is put on with a brush having stiff bristles. The chemical effect is in changing the caustic lime into calcium sulphate or gypsum, and zinc oxide is formed in the pores of the cement. This treatment has no injurious effect on the cement surface; on the contrary it makes it fit to take and hold the paint. This method of preparing cement for paint has been well tried out and is reliable.

The following formula for painting on concrete or cement may safely be followed: For the first or priming coat, after the sur- face has been sized with the zinc sulphate solution and is dry, mix ioo lb. pure car-

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