This is a document that was written by a dear friend of mine on the internet back in 1994, as a part of his cultural anthropology studies. He gave it to me to use, and you may quote it, provided you give proper credit to the sources.
Author: Jonathon Kerr, Queens University, Belfast, Ireland
Casting a Late Bronge Age Sword
by Jonathan Kerr
When casting a sword, the first thing I need is a mould. This would be carved out of stone, and be based on an example of a sword I already have. If I have no sword to hand, I would fashion a model from wood, and decorate this, in relief as a template for my mould. I would use either soft sandstone, which could be easily worked with a bronze chisel, such as York Freestone, or make a clay mould, a rather more technical technique, but easier procedure. I would be supplied with metal ore from one of the local Irish mines-- copper was widespread, with only a noticeable absence in the centre of Ireland, but tin was harder to acquire in Ireland, with only one source at Dublin, making bronze weapons extremely expensive outside Dublin, with their cost increasing as the distance from Dublin increased. Tin was also imported from Scotland, increasing availability, but expensive nevertheless.
I would use coarse, sandy clay and spread it out along my worktable, longer and wider than the sword I hoped to construct, then sprinkle it with a finer clay. I would then take either my wooden template or the sword I had already had and gently impress it on the clay, until it was half buried. Leaving the template in place, a layer of find sand or ash was sprinkled around the edges of the mould to prevent the two halves of the template from sticking together. The second half of the mould would then be made by pressing the same mixture of fine clay, followed by strengthening coarse sand and clay as used for the first half onto the pattern, and shaping it similarly to the first half. I would then build a straight stick onto the length of the second half of the mould, so that it can be removed later without distortion. The stick would be removed later when the mould was drying. Using the stick, I would then remove the top half of the mould, and remove the template, allowing me to trim the mould and tidy it up. Finally, I would cut the jet - the funnel-shaped surface over which the molten bronze would be coated with soat, to ensure that the metal is properly oxidized. The mould would then be set aside to dry. When the mould had dried, it would be fired to a brick-likeconsistency, ready for the casting.
Before the melt can be added, the mould must be heated to a temperature between 200 and 300C, to ensure that themelt does not solidify before reaching the bottom of the mould. The heating of the mould, be it stone or clay, can cause some warping of the mould, meaning that the two halves would not be a perfect fit. It may actually be preferable that the two halves of the mould are not a perfect fit, because it allows air to leave the mould cavity. Also, a bad fit between the two mould halves effectively thickens the thinner sections of the mould, allowing the melt to flow more easily, while if the two halves were a perfect fit, the result may be a mis-run, with the melt solidifying too early. As long as the gap between the moulds is not too large, the amount of flash (excess metal laying in the gap between the mould halves) can generally be kept to a manageable minimum. The critical point is to get the minimum loss of metal, a factor determined by the smith's experience.
The melt itself would be a critical factor. Getting the ratio of tin to copper is critical. The amount I would add would be 13.6% tin (the mean value for the middle bronze age). This is the limit of tin that can be dissolved in copper.
The mould would be buried in the ground, surrounded by hot sand, with the top and the jet just above ground level, ready for pouring. After pouring,, the mould would be removed from the ground, and if still intact would be reused, although clay moulds tended to be cracked after one use.
The sword would now have to be trimmed, removing the flash and the funnel shapped imprint of the jet, as well as being hammered to sharpness. First it would be scrubbed clean, using sand and water, and smoothed down with gritstone. To sharpen the sword, it would first be homogenized by heating it in a charcoal hearth, making it more pliable to facilitate forging. It would be hammered with the opposite edge supported with a stop to prevent the sword from 'running' under the hammer.
In conclusion, there are no real metallurgical problems casting bronze since it has a low melting point, and is easily worked. Generally using stone moulds, shaped with bronze tools, is much simpler and less problematic than using sand moulds. The most important factor isthe ratio of venting to metal loss. The critical area, and the first to suffer if venting is insufficient is about 2/3 of the way alongthe blade, when the metal is poured from the tip. One way of combatting this problem is to thicken the mould in this region slightly, meaning there would be less difficulty in running the melt.