It is, therefore, usual to have more vents than jets, and they should be so placed that wherever there is a bell or pocket formed by the configuration of the mould in which the air might be imprisoned, there must also be a vent provided for its escape. It may seem strange, but greate#attention is needed in this respect in small than in larger works, because in a large work the metal does not set so rapidly, and thus gives time for the air to escape into the pores of the mould, whilst the pressure to do so is much greater. In any case the labour of placing these jets and vents is but small, and as their importance is paramount, the best advice always will be "When in doubt put a vent - put a dozen rather than run even a small risk."

Piece-moulding in loam does not require the same attention in this particular, as the joints in the mould act as a complete network of vents. This seems a very great advantage over the wax process, and so it would be were it not that the method of placing these jets and vents is so easy and simple that it involves but little skill, and is easily done by an assistant. The artist need not lose his time over it, beyond defining their positions and relative sizes.

The jets and vents, being merely a system of tubes leading from the outside of the mould to its inner cavity, become of course filled with metal, which remains as solid rods attached at one end, while the other end is free in the air. • All that is therefore needed is to make rods of wax of the required size, and attaching them by one end in their proper position on the surface of the work, bend them into the required curves. As, however, it would be inconvenient to pour through more than one jet, it is usual to join them together in groups, and to bring these again together to one main jet, which leads directly from the basin, or "sow," into which the bronze is poured. By this means, the bronze flows from the main jet through all the subsidiary ones to the different parts of the mould.

The vents are treated in exactly the same manner, except that they do not usually end in one main duct but in several, as may be most convenient, nor do they rise to the basin, but come to the open air at the top of the mould, outside of the space occupied by the pouring basin.

Besides the jets and vents there are other rods of wax which are called drains or spouts'; these are placed on the lowest portion of the work, and lead through the thickness of the mould, but slanting a little downwards to the open air. Their object is to drain away the wax where it is melted out, after which they have to be securely stoppered up. In very small works drains are entirely omitted, and the mould is merely turned upside down to allow the wax to run out.

There are two principles of pouring bronze, each of which has its advocates. The one is the descending and the other the ascending principle. In the former the bronze is cast directly into the cavity from above, as shown on Fig. 198, which is not designed to represent any particular work, but merely to illustrate a principle. By this method, the bronze enters through all the various jets,and gradually fills the mould until it reaches the top. In the second or ascending principle, as shown on Fig. 199, the jets are taken right down to the bottom of the mould before they are allowed to enter its cavity. The metal of course rises to its own level, thai fill-ing the mould from the bottom.

The disadvantages of the descending or direct method are these. When the bronze is cast, it flows directly into the trickle over the core and over the surface of the mould, making a great amount of atmospheric disturbance in the cavity. These streams unite at the bottom, and then the mould begins to fill after the air contained in it has been heated and expanded to the utmost; nay, it is more than probable that portions of air caught between two metal streams may be dragged along and forced to escape by bubbling upwards through the molten metal, causing the latter to boil and work, and thereby endangering the delicate and frail inner surface of the mould. Moreover, the streams of metal, having united at the bottom, must rise and pass again over the surface of the mould before it can be completely filled. In the ascending principle, the streams of metal pass harmlessly to their lowest level before entering the cavity of the mould, thus its surface cannot be injured by the downward rush, and the air is not brought in contact with the metal until it begins to rise in the mould.

No particle of air can become entrapped, for it is all above the surface of the rising bronze, with a free exit through all the vents above, through which it is forced by the pressure of the rising bronze, and by its own enormous expansion, which, however, cannot be so great or so sodden as When the metal enters from above, rherefore, always cast on the ascending principle.

Descending praam.

Descending praam.

Metal Work Brozes Part 5 500136

The jets and vents having been arranged with due consideration, it becomes necessary to think of the mould, and to consider the qualities to be sought for in a mould for bronze casting. In the first place, the material used must be one that is easy of application, and of a nature to resist the intense heat to which it must of necessity be subjected. It must also not be antagonistic to the metal, which would otherwise be spurted violently from the jets and vents, besides which it would certainly yield a blunt and bad impression. There are many recipes for such moulds extant; almost everyone who practises this art has his favourite mixture, concerning which much secrecy is observed. There is, however, no great secret about this matter. Many of these moulds, especially those of large size, which, of course, have to endure the most heat, are made of luto, or potee, as it is indifferently called. Thjs is nothing more than a mixture of which the basis is fine loam. The loam may be natural or artificial; the whole secret consists in purifying it sufficiently, and in grinding it very fine before use.