< ENLARGEMENT
CROSS SECTIONAL ENLARGEMENT
Though not quite as accurate a method as pointing, the cross sectional enlargement technique does have the advantage of being relatively quick and straightforward to execute. The cross sectional method is also well suited to use with direct plaster modelling, which in turn makes it possible to supply the completed enlargement direct to the foundry as a master pattern, with no need for waste moulding.
One of the disadvantages of this technique is that it normally requires the use of a ‘sacrificial’ scale model, which is effectively destroyed during the enlargement process. If the model is important and needs to be retained for later reference, an exact copy can usually be obtained by creating a rubber reproduction mould of the chosen maquette, from this mould a technician can produce a plaster copy which in turn can be used for the enlargement. Alternatively, it may be possible to plot the maquette’s cross sections using a stylus and pen arrangement.
Once a sacraficial scale maquette has been produced, it is divided and cut into convenient sections, each numbered for future reference. The maquette is normally sectioned into a series of evenly spaced transverse divisions, though if more appropriate the model can be divided longitudinally. The maquette should be divided into enough sections to give an accurate impression of the form, but not so many as to over complicate the process. The divisions should also be easily scalable, allowing a stock thickness of expanded polystyrene foam (EPS) to be used for roughing out the enlargement. For example, if the ratio of enlargement is 1:5 and the scale model is sectioned into 3/8 in (10mm) divisions, readily available 2 inch (50mm) thick cut foam blocks can be used for the pattern’s core construction.
The maquette is cut through it’s marked out divisions, taking care not to loose too much detail in the cut itself, or deviate from a straight line – the use of a fine band saw can assist here. If necessary, a common registration mark can be included to allow easy the reference of each piece with it’s neighbour. Once all the divisions are cut the model is reduced to a series of equal slices which, if stacked one on top of the other, restores the model.
The next part of the enlargement process requires the outline of the individual sections to be accurately traced. This can be done manually by drawing around each of the model’s cut profile, or input into a computer either by scanning or plotting each profile. Manually drawn profiles can be enlarged by transferring onto a grid scaled paper template, though more commonly a drawn profile is sent to a good print copy shop and enlarged via a professional level photocopier. Computer scanned profiles can be scaled using a suitable drawing or CAD programme, with the result printed out by a large scale stylus plotter onto paper, or else input directly to a CNC compatible machine which can then cut accurate template profiles.
Once a scaled drawing has been obtained for each cut section, the profile of the model can be transferred to a proportional thickness of polystyrene (EPS) foam or similar material. Strictly speaking the transferred drawing should be made marginally smaller than a full enlargement, this allows for a skin thickness of plaster to be added later – though in practice most artists will ignore this allowance as a relatively insignificant deviation.
There are a number of ways to transfer the scaled up templates to foam blocks, but one of the most common and straightforward methods is by ‘pricking. This technique involves a series of small perforations being made through a paper template, following the line of the drawn profile (the end result is similar to the perforations dividing stamps on a sheet). Each perforated drawing is secured onto it’s own foam block, then dabbed with a muslin bag loaded with finely crushed carbon black, dark pigment or a similar indicator. The dusting transfers through the template’s perforations to the foam’s surface, showing up as a series of dots once the overlaid template is removed. The dots can then be joined to redefine the profile of the enlarged section.
Once a template drawing has been transferred to it’s corresponding polystyrene block, the ‘pricked’ line can be used as a guide for fashioning out the profile that particular section. This can be done by first cutting the EPS block with a battery powered hot wire. The series of cut foam blocks can now be stacked in order and glued together to create a rough, but enlarged version of the original model. The rough polystyrene reconstruction can then be refined using a cheese grater, coarse rasps and other abrasive tools or grits.
The finishing procedures from this point on are very similar to those used for the construction of direct plaster patterns over foam cores. After the foam ‘core’ has been checked for accuracy and secured, plaster dipped scrim is laid over the core’s surface, taking particular care to ensure adequate adhesion between the foam core and plaster. Surface detail and textures can now be worked into the enlarged pattern by overlaying the scrim bonding with a finishing layer of fine plaster of Paris. This final layer can be built up, modelled, carved and worked by the artist or technician, using the original model as a reference to ensure an accurate reproduction of forms and surface textures.
DIGITAL ENLARGEMENT >
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