< RECONSTRUCTING CAST SECTIONS 1
RECONSTRUCTING CAST SECTIONS 2
1. The abutting edge of each cast section is cleaned, This is carefully done, ensuring no investment or sand refractory remains in the area of the joint. Unless the cast has a wall thickness of less than 1/16 th inch (3mm), each joint edge is bevelled to a 30° angle, This produces an 60° INCLUDED ‘V’ shaped WELD PREPARATION when the two sections are offered up to each other. Some chasers prefer to bevel weld edges after the cast sections have been aligned and TACKED in position.
2.The sections to be joined are OFFERED UP to each other with their respective registration marks carefully aligned. In some cases it may be appropriate to use a JIG or a similar device, to keep individual cast sections in their correct relative positions. A jig can also be kept and used again to maintain common accuracy when multiples of the same design are being reconstructed.
3. Sections for welding are first tacked together in a sequence determined by the chaser. Unless there is good reason to do otherwise, individual sections are rarely fully welded until all the separate elements of the work (arms, legs, head etc) have been positioned to the satisfaction of the artist. Tacking allows any placed section to be adjusted with relative ease to achieve an overall correctness in the unified cast.
Large scale designs are usually constructed with the aid of lifting equipment, supports and other similar devices. These items are variously designed to assist the chaser and make for a safe fabrication. If the work is viewed at this stage by the artist, it is important to try and ignore the visual distractions created by these supports and to focus instead on any correction of the sculptural elements.
4. A slight mismatching of offered up sections is not uncommon. Mismatching can happen for a variety of reasons, but is normally a compound effect of differing shrinkage rates – in the rubbers used for moulding the master pattern, the wax used for the wax pattern production and later, metal contraction in the refractory mould – all of which may combine to slightly pull a cast edge out of alignment with it’s neighbour. A slight mismatching of cast sections, whilst undesirable, is usually of minimal concern as most of the copper casting alloys used in art founding are easily manipulated back into shape by COLD WORKING with a soft mallet.
It is important to note here, that few of the common casting alloys used by art founders are HOT WORKED in the manner of forged steels, the exception being some SILICON BRONZE and BRASS variations. The manipulation of other copper based casts at red heat can result in the tearing of the cast’s fabric – this is especially the case with GUNMETAL type alloys. Cold working of a cast gunmetal is possible with the aid of hammers, punches, hydraulic jacks and so on, though care should be exercised not to damage the delicate surface of softer alloys. Some cast alloys will structurally harden when cold worked and these materials may require NORMALISING or ANNEALING to prevent stress cracking. Some of the more common lead bearing cast alloys (leaded gunmetals and leaded brasses), are quite resistant to the effects of work hardening.
Where a more severe manipulation of the cast is required, jigs and mechanical jacks can be used to work a distorted section back into it’s correct alignment. A badly distorted cast may be partially cut along it’s wall thickness to relieve stress and to allow sufficient movement in the distorted edge to pull it into shape (see PDF). The manipulation of cast irons is more problematic because many cast iron materials will easily fracture if cold worked. Considerable care needs to be exercised when aligning sections formed in this material.
5. Once correctly positioned and secured by tacking, the cast work can be visually checked against the master pattern. If the pattern is measured by rule for comparison, remember to make an allowance for contraction in the cast version of the design. It may be advisable to have the artist to approve the work before it is fully welded, especially if the design has been fabricated from a significant number of separately cast sections. The founder will usually request the artist’s attendance if viewing is desirable.
RECONSTRUCTING CAST SECTIONS 3 >
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