This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
In examining castings, with a view to ascertaining their quality and soundness, several points should be attended to. The edges should be struck with a light hammer. If the blow make a slight impression, the iron is probably of good quality, provided it be uniform throughout. If fragments fly off and no sensible indentation bo made, the iron is hard and brittle. Air bubbles are a common and dangerous source of weakness. They should be searched for by tapping the surface of the casting all over with the hammer. Bubbles, or flaws, filled in with sand from the mould, or purposely stopped with loam, cause a dulness in the sound which leads to their detection. The metal of a casting should be free from scoriae, bubbles, core nails, or flaws of any kind. The exterior surface should be smooth and clear. The edges of the casting should be sharp and perfect. An uneven or wavy surface indicates unequal shrinkage, caused by want of uniformity in the texture of the iron. The surface of a fracture examined before it has become rusty should present a fine-grained texture, of an uniform bluish-grey colour and high metallic lustre. Cast-iron pipes should be straight, true in section, square on the ends and in the sockets, the metal of equal thickness throughout.
They should be proved under a hydraulic pressure of 4 or 5 times the working head. The sockets of small pipes should be especially examined, to see if they are free from honeycomb. The core nails are sometimes left in and hammered up. They are, however, objectionable, as they render the pipe liable to break at the points where they occur.
As there is an endless variety of patterns from which moulds are made, it will be necessary to divide them into light and heavy work. Stove castings are very light. In the moulding of such work, much depends upon the quality of sand used; the moulders' heap should be composed of no more than 1/2 loam, the other 1/2 being a very open sand. This makes a good strong mixture, which will not allow the sharp corners and fine ornamental work to be washed away when the molten iron is poured into the mould. In ramming such work, the moulder should be careful that the sand on top and bottom of his pattern is not rammed hard; but the sides or edges should be well rammed, in order that the casting may not strain from having a soft parting. Great care should be taken to see that the bottom board is well bedded on the flask, after which it should be removed and the vent wire used freely. The venting of the work is often but partially done, on account of the point of the vent wire coming into contact with the pattern; and when the iron enters the mould, it finds its way into said vents, fills them up, and thus, in a measure, prevents the escape of the gas that arises from the iron coming in contact with the charcoal, graphite, or soapstone with which the mould has been dusted to prevent the sand from adhering to the casting.
The bottom board should then be carefully replaced on the flask, and dogged down so that in the act of turning it over it cannot move, which would cover the vents over with sand. The top part of the flask (or cope, as it is termed) needs the same care in ramming over the pattern as the bottom, and should be well vented. If the mould has any high projections in the cope, they should be well vented; for it is at these elevated points that a large portion of the gas accumulates and needs a quick exit, in order to make sharp corners on the casting and prevent blowing. The strainings of castings in this branch of the trade is greatly due to an insufficient amount of weight being placed on the flask, or the parts not being properly dogged together, as well as to the rapidity with which the iron is poured into the mould, together with the height of the runner. Cutting short the supply of iron as soon as the runner is full, and a careful watching of the work to be poured, will in most cases remedy the trouble of the casting being thicker than the pattern.
As to the warping of the plates, much depends upon the quality of iron used and the judgment of the pattern-maker. It can often be prevented, in a measure, by the moulder, in making the runner from the round sprue no thicker than the piece to be cast; and as soon as the metal is poured, by digging away in front of the sprue and breaking it loose from the casting. Where a flat sprue is used, this breaking off should invariably be done as soon as the runner is cool enough. Being wedge-shaped, with the small end of the wedge downwards, it lifts a portion of the casting in shrinking, and thus causes it to be out of shape.
In heavy work, care and judgment are needed, and it requires a man's lifetime to become proficient. In ramming work that is to be poured on its end, having a height of 3 or 4 ft., there is no risk in well packing the sand, for 2/3 its height, around the pattern; and as you near the top, ram it as you would a pattern no more than 1 ft. in thickness. The sand in all such work should be very open or porous, in order to prevent scabbing. As there is so large a quantity of iron used, much steam and gas are generated in the mould; and as there is no other way of escape for them but through the vents, there should be no fault in this particular part of the mould. In the pouring of such work, it is best to run it from the bottom. If a runner is used, do not raise the risers to correspond in height with the runner, as by so doing you increase the amount of strain on the mould; but form a little basin around the risers by ramming out the sprue holes with the finger, and on the side nearest the outer edge of the flask form a lip for the surplus iron in the runner to run over on to the floor. When heavy work is bedded in the floor, too much care cannot be taken in preventing the dampness of the ground beneath from striking through into the mould.
The sand that is thrown out of the pit, if it has been of long standing, should not be used for the moulding of that piece; for it is too cold and damp and should be thrown on one side, and allowed to stand, that it may dry and warm up. The 2 or 3 ladlefuls of iron that remain in the furnace after the work on the floor has been poured, can be run into pigs in this sand, which will greatly help to fit it for immediate use. In the venting of heavy work, the small vents should terminate in a number of large ones, which should have an opening on both sides of the mould : then a draught would be formed to carry off the gas which is continually growing as the workman is in the act of pouring the iron into the mould.
All men connected with this branch of the trade have heard that sharp report which immediately follows the pouring of a large piece, and which is caused by the confined gas in the lower end of a large vent, there being no draught to drive it out. Where facing is used, much more care is needed in venting. In the making of large pulleys and gear-wheels, too much care cannot be taken in this particular. Not so much depends upon the ramming of such work as upon the venting for the proper exit of the gas from the sand in the immediate vicinity of the mould; for if the mould has been rammed harder than there was any necessity for, and the venting has been properly looked after, there is not much danger of the casting being a poor one. Such work should invariably be run from the hub or centre, with sufficient risers, arranged as above described. This branch of the trade is called green-sand work, and it involves a large part of the art of ramming.