Delft casting® technique
General considerations for the Delft casting technique
Objects used for champlevé work usually need to be thin. This is feasible if one takes potential pitfalls into account. The amount of silver used is small, and so is the volume to surface ratio; hence it solidifies quickly in clay. Any irregularity in the mould that obstructs the flow of silver (such as grains of clay, bulging edges of vents and casting gate, flowing against gravity, narrow vents, etc.) in principle have major influence on the resulting cast. Here are some general recommendations for the Delft casting technique.
- See to it that the space in which you do the casting is well ventilated; otherwise you may develop a headache.
- Use a magnifying glass to inspect the polymer mould. Are the edges of the cloisons of equal height and width, are they smooth, are the depressions sufficiently deep for smaller details? If not, it is best to make a new mould as results are otherwise likely to be unsatisfactory.
- Use double-sided adhesive tape to fix one or more layers of acetate sheets (or thin metal sheets) to the back of the polymer mould), so as to increase the minimum thickness of the photopolymer mould to 0.7-0.8 mm for small objects, thicker for larger objects. Use scissors to trim the photopolymer mould and acetate backing.
- Carefully remove dust and dirt from the site
where you are going to use the Delft clay
.
The surface should be clean, flat and very smooth. - Chop the Delft clay finely with the edge of a thin metal ruler; the better it is cut up finely, the smoother the surface into which you will press the photopolymer mould. It also prevents the formation of air sacs, which might expand during casting and thus ruin the cast.
- We need only pay attention to one side of the photopolymer mould: after all, the back is flat.
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| Figure 11 - Put the ring with the mould facing down on top of it between two inflexible metal sheets and compress the whole with a gluing clamp. |
Preparing the mould
- Put the ring with raised edge upside down on a flat and even surface. If the surface is not smooth, then put the ring on a sheet of acetate, so that ultimately the mould will be pressed into a smooth clay surface.
- Fill the ring half with clay, firmly compact the clay with your fingers. Subsequently hammer the clay firmly. Then fill the ring generously with clay so that it is overfilled, and compact this firmly with a flat hammer again.
- Remove excess clay with the edge of the metal ruler, then turn the ring so that the raised edge now faces up.
- Powder the clay and polymer mould thinly with talcum powder; blow excess powder off. Apply the powder thinly so as to avoid it giving rise to pits in the silver object. There is no objection to using the ball of the hand or a metal ruler after powdering, to obtain a smooth surface free from unevenness and granular structure.
- Put the mould face down on the smooth clay surface. As the mould (made of UV sensitive photopolymer, Jet®) is still quite flexible, put a thick metal ruler on top (so that the mould remains flat) and press this firmly into the clay. If you have hammered the clay down very firmly you may find it difficult to obtain a deep impression. In that case proceed as follows (figure 11). Support the ring with a flat and inflexible piece of metal, put a similar piece of metal or a thick metal ruler on top (i.e. on the mould's rear side),and compress the whole with a gluing clamp. Now remove the clamp, leaving the mould in place.
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| Figure 12 - Put the second ring on top of the first one, lining up the grooves. Fill it completely with clay and then hammer it down firmly. |
Then proceed as follows:
- Put the second ring on top of the first one, and see to it that the grooves line up. You may have to lightly powder the clay and the back of the mould; fill the upper ring with clay and hammer it down, as you did previously with the other ring (fig. 12).
- Wriggle the two rings apart and carefully remove the mould, e.g. with the thin edge of a knife. Check that the print is complete and sharp.
- Put the upper ring upside down on a flat surface, so that the side facing the mould is up. Stab vents (figure 13) by pressing a thin, hollow tube (use e.g. disassembled parts of an old telescopic car aerial), lift the ring with the tube in place, press the tube a bit deeper and remove any protruding clay with a knife. Remove the hollow tube, if need be slightly widen the opening with your knife, and put the ring on the table again. With small objects the vents should preferably not run vertically but from inside out (fig. 14), leaving enough space for the casting funnel. Move the hollow tube straight down in one go, do not wriggle it. Proceed to prepare more vents. The vents should be placed at or near the edge of the mould; after all, all the air must escape, and this will not occur if the vent is centrally placed and fills up with silver before the mould's edge.
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| Figure 13 - Provide the clay in the upper ring with vents and a pouring gate. |
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| Figure 14 - Cross-sectional view of vents, casting funnel and mould. |
- Use a hollow tube of at least 5 mm (0.2 in.) internal diameter to prepare the casting gate (figure 14); using one such tube you can enlarge the gate to a slightly oval form by two overlapping stabs. Make a vertical stab without wriggling as the latter would displace clay at the casting gate, leading to a protruding and brittle edge. Use a pointed knife to widen the stab to a wide funnel, leaving only a minimum of the casting gate nearest the mould. A spacious, wide funnel results in greater silver weight and volume and will help the silver to flow farther before solidifying. Using the hollow tube or anything else carefully flatten the funnel's surface, including the pouring gate (inset figure 14): the straight edge is not quite stable so that clay particles may become dislodged during casting, causing impurities in the casting. Subsequently remove any loose particles, check whether the vents are still clear, and gently blow through the funnel.
In the example in fig. 14 the funnel is located centrally above the mould. This is useful in the case of larger moulds, say 4x4 cm (1.57x1.57 in.), reducing the number of flawed castings; it does entail some extra work later, however, in levelling and polishing the rear side of the object. With small moulds the casting gate and the vents can be prepared at the edge of the object, or even further away, with wide channels leading to the mould.
- Remove any clay from the edges of the two rings. Put the mould face up in the upper ring again and press it gently, in particular at the casting gate, as its clay floor may have become somewhat depressed by the various manipulations. This is because in preparing the vents and the funnel the clay may tend to protrude. This will impede the flow of molten silver, hence cause nonhomogeneous or incomplete filling of the mould and lower edges of the cloisons, as well as causing other imperfections. This is because with flat moulds the mould is quite narrow, hence the volume to surface ratio very unfavourable; every flow impediment, including too few or too narrow vents, will then cause the silver to solidify prematurely and the casting to be imperfect.
- Subsequently remove the mould and assemble the two rings, carefully aligning the grooves.
- Put the assembled rings on a flat surface where you are going to do the casting. See to it that they are level, in particular if the mould is very thin. You can check whether a flat surface is level with a marble or spirit level. Some imperfections in the final cast are due to the rings not being level, in particular if the temperature of the silver when casting was not optimal and/or the vents were too narrow or too small in number. The reason for this is as follows (figure 15).
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| Figure 15 - Even a small inclination causes the molten silver to flow a few millimetres against the field of gravity from the casting gate. The lowest parts will fill up first, the highest ones last, and by that time silver solidifying prematurely may lead to a flawed casting. |
Suppose you want to cast a round object that is 0.8 mm (0.314 in.) thick (excluding the edges of the cloisons) and 5 cm (2 in.) in diameter, and that it makes a 5° angle relative to the horizon. In that case one edge is 2.18 mm (0.086 in.) above, the opposite edge 2.2 mm above the mould's centre (25 mm x tg 5° = 2.18 mm = 0.086 in.); the difference in height in this case comes to over 2½ times the thickness of the object. As silver flows preferentially to the lowest parts, in case of any impediment it may have cooled off too much to completely fill up higher parts. This is why the vents need to be sufficient in number and sufficiently wide, so that the quickly expanding air can escape (the gas expands 4-fold in a fraction of a second) allowing the silver to flow.
- See to it that the silver and its container are properly heated when casting. The most common reason for a casting not being perfect is that the silver was not really hot. When you are ready, position the container on the rings and pour all the fluid silver in one fluent and quick movement into the casting funnel while you continue to heat the silver with the flame.
- Immediately after casting hold the rings under cold running water to cool the silver.
- Wriggle the rings apart, dispose of all burnt (black) clay and store the remaining clay in an airtight plastic bag or box.
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Figure 16 - All black parts of the figure on the left will be depressed in the mould, which will be thinnest there. The white lines give rise to the edge of cloisons. With a centrally positioned casting gate (i.e. at the hands) there is a chance that the lower part of the ellipse, and the upper part with the eye for the chain may not fill perfectly in the object to the left. There is a similar risk in the right object, as the greatest proportion of the silver needs to flow through the flattest central portion to the periphery of the ellipse; you would improve your chances of a successful casting by making it thicker or positioning the casting gate excentrically (to the right, in the white part). |