G88
THEORY AM) PRACTICE OF ENGINEERING.
Rook 11.
FLASK CASTING.
'fhc sand is usually passed through a sieve, and the surface made hard and level bycontinuous heating with a rammer, and then smoothed by a trowel : an edge of the samematerial to coniine the casting is formed around it, by first laying laths of the thicknessof the intended plate on the prepared bed, and then making the border or edge close tothem ; the laths are then removed, and the metal is poured into it. To prevent the castingfrom cracking as it cools, the moment it sets it is covered with a thickness of 3 or 4 inchesof dry sand.
Flask or Box Castings require a more perfect model or pattern, which is usually madewith boxes or flasks as they aretermed, either out of wood ormetal, put together in such amanner as to admit of beinglengthened or shortened, and thusadapted by bolts and screw fasten-ings to an endless variety of formsand sizes. To make a castingtwo boxes are required; one isplaced nearly on a level with thefloor of the thundery, which ispreviously covered with sand andwhich forms the bottom : intothis is put the moulding sand,which is well rammed in until a sufficient bed is made for the introduction of thepreviously prepared pattern, the centre line of which should about range with the top ofthe box ; the whole is then filled up level with moist sand; this being pressed or rammedin and made smooth at the top, a quantity of perfectly dry sand is sprinkled over thesurface, to occasion a parting or separation when the upper box is placed upon it; beforethis is filled, it is necessary to provide a vent for the escape of the air when the metalis poured in, and for the gate, a funnel-shaped channel that is to admit it: these twoopenings are preserved by placing two upright rods, slightly tapering, in the positionrequired previously to filling the upper box with sand, which when rammed in, the boxis to be lifted up and the pattern taken out, leaving its impression in the sand : thisrequires some skill and adroitness on the part of the workmen, who, by moistening thepattern with a little water, consolidate the sand immediately around it, and its liberationis rendered easy ; should any breaking down of the sides of the mould take place, it musthe repaired and made perfect. When the casting is to be hollow, as in cylindrical pipes orcolumns, it is necessary to introduce a core which shall corre-spond in dimensions with the void required, and the formationof this core demands considerable skill: a core barrel is madeby twisting hay bands round an iron column, wetted and houndcarefully from one end to the other; this is covered with well-tempered wet loam mixed with cow hair ; were clay used itwould burn into a brick or hard substance. When the coreis thus prepared, it is rendered true by turning in a lathe, afterwhich it is hardened in a stove or oven constructed of bricks withracks or shelves in it, and closed with iron doors: when thecore is taken out, it is covered with finely ground coal-dustmixed with water, and when dried it is ready to introduce intothe previously prepared mould ; the heat of the metal pouredin burns away the hay, which separates the iron core from itscoating of loam, and when the iron begins to set the barrelis withdrawn, and the loam is scraped or chiseled out.
The process adopted by John Weicliard Valvasor, of Carniola ,of casting statues very thin is given in the Philosophical Trans-actions , vol. xvi.: he first formed with good clay that enduredthe fire, and would not crack either in drying or burning, thefigure or statue to be cast: when the model was quite dry, smallholes of moderate depth were made over its entire surface, intowhich were placed small pieces of metal, to keep the core andmould from touching each other, or from falling together; aportion of the clay was then scraped away to as much as was in-tended to constitute the thickness of the statue, and the mouldwas placed in a furnace and heated red-hot: when cold, it wasrubbed over with an earth used by the German potters to colourtheir tiles, resembling black lead, for the purpose of making themetal flow freely over it; yellow wax, mixed with pitch or resin, Fig. 500. statue casting.was then spread over, to the thickness of the metal to be given to