At the request of some friends, a project to recreate the Loshult gonne is as period a manner as possible has been started. While the internet and variousreinactment circles are replete with variously manufactured copies of this, one of the oldest surviving bronze gonnes of the medieval period (1326),few reproductions follow the methods utilized by medieval bronze founders. Utilizing the texts of Theophilous (1122) and Birringuccio (1540), as well as archaeological evidence of bellfounding,we can piece together a fairly complete picture of early bronze casting efforts in ordinance manufacturing.Presumably this vase-shaped bronze gonne of approximately 25 lbs would have, in the 14th century, been produced much in the manner of other large vesselsfamiliar to the founder, particularly bells, and not as a sand casting or turned from solid stock as is done modernly. While these methods have their place and can produce a similar looking piece, that is not the focus of our project.
As the technology to bore cannon barrels from a solid mass was not properly achieved until the 18th century, thecore would have been integral to the casting, held in place by iron pins as in the case of bells, statues, or any other cored object. The art of securingthe bore core in perfect alignment with the barrel while casting would have been among the many secrets of the master founder, and while Birringuccio describesseveral methods for securing the cores of large cannon, there is room for a wide range of interpretation when it comes to putting it into practice, and themethods used on cannon weighing several tonnes does not necessarily translate to methods used on hand gonnes, harquebusse's, and the like. Regardless of the exact method, it is certain that the core was cast in place, if not on the basis of standard foundry practice but on the fact that the bore is not cylidrical but chambered and flared. The only straightforward means of producing this shape would be again a bore core, tooled externally and cast in place. The vase-like shapeof the gonne also lends itself to production on a lathe in wax or tallow, turned in the same manner as bells, described in detail by Theophilous. While Birringuccio's method of cannon construction (building up of a clay jacket over a wooden model which was greased with tallow to allow logitudinal removal before firing the mould) lends some interesting details, the particular design of the Loshult gonne would tend to favor the earlier method. His method of inserting the bore core, affixed by a chaplet ring at the breech and a disc at the head, has particular merit which solves issues of having to turn the model on the same spindle as the core is attached to. By combining details of both these accounts, pre- and post-14th century, as well as common sense, we come up with a workable solution for construction.
All accounts of medieval, as well as ancient and up to fairly recent modern, lost-pattern casting involve the use of a fired loam, essentially clay modified with organic binders and sometimes refractory material, that is built up in layers over the model which is subsequently fired to remove the model and cast. Regardless of the particularformula (Theophilous doesnt give one, Birringuccio gives several, and modern Ashanti craftsmen have a different mix altogether) the properties of the loam *must* be that it reduces the shrinkage of the mix to be able to dry and fire withoutcracking apart, and it must be alble to stand the thermal shock of the pour, while retaining some degree of acceptable surface finish. Whether the organic binder is horse dung,flax seeds, rice hulls, wool clippings, or grass and whether the refractory material is sand, chamotte, wash ashes or charcoal dust, the final clay content can rarely exceed 2/5ths of the mix by dry volume. The exact porportions and application are again the individual founders secret, however this give us a workable place to start. The methods of layering up the moulding loam range from applying bats of moist but semi-firm mixture to brushing it on as a paste. In the end, the successful proprietary mix will give the best surface finish and the fewest number of casting defects such as blow holes and flashing, however *all* bronze casting require some degree of cleanup as part of the process, and the finishon cast pieces today rarely reflects the appearance they had when broken fresh from the mould.
For melting large quantities of bronze, both our primary sources as well as other contemporary accounts describe, among other methods, a type of shaft furnace where metal and fuel intermingle and are fed with forced air from bellows at its base, to be pooled in a molten state and tapped out when ready. While a reverbatory furnace on the other hand was generally the favored method of the cannon founder in the 15th century and beyond, it relied primarily on existing scrap bronze, whose significantly lower melting temperature could be achieved easily but rarely exceeded in this type of furnace. The initial alloying of the bronze however requries a much higher temperature by several hundred degrees F in order to first liquify the copper before adding tin, and this is more readily achieval in the direct fuel-metal contact of the cupola or shaft furnace, which could also achieve temperatures hot enough to produce cast iron (Birriguccio in fact alludes rather humourously to the futility of trying to melt copper in a reverbatory furnace). So, our current setup for melting and alloying the requisit 40 lbs of copper and tin into bronze will be a 10" cupola furnace.
Materials
- Charcoal - crushed and screened to 3/4'" - 1 «", approx. 20-30 lbs
- Copper - aprox 40 lbs
- Tin - 5 lbs
- Wood - hardwood for starting cupola and firing mould, available free from scrap yard
- Wax and hardener - beeswax, may need a hardener in order to be tooled
- Sand - fine sand for the mould, one bag
- Dung - dried and shredded, for mould, large tub
- Clay - 50# bag of ball clay
- Rope fiber - for outer mould layers. Needs shredding.
- Mandrels - 1 of iron for the bore core, one of wood for body
- Binding wire - for mould, 2 sizes