archaeometallurgy

Sand moulding, or rather the casting in naturally bonded sand can be traced back to 17th century AD originating in France. Modern casting industry’s demand of constant quality of casting sands led to a decline in the use of  sand which is bonded naturally.  A few art foundries in Germany, though, are still knowledgeable therein and use it for casting purposes products. Due to the high expenditure of human labour it is only a question of time until all art foundries will work with the lost-wax process.
The main difference to modern casting sands are the utilized binders. In naturally bonded sand this is usually a percentage of clay. In industrial sand they are Bentonite (mainly a clay mineral called Montmorillonite), Furans, Phenolic resin or similar chemical compounds. Since this page is concerned with archaeometallurgy we will dismiss all the explanations on modern casting technologies.

The stop motion animation shows the fabrication of a green sand mould with naturally bonded sand. It is a reconstruction of a Zeiss Junior microscope foot. This mould was made for aluminum casting.

Moulding with naturally bonded sand

Since sand on its own is not enough to produce a sand mould, a frame for holding the sand in place is needed. In foundry terms this is referred to as a flask. Flasks are usually rigid frames without fixed bottom or top and consist of two parts: a lower one also known as drag and an upper one which is called cope. So called flask pins at the sides of the cope and drag ensure a proper alignment of upper and lower half during moulding and later, once the pattern is withdrawn, during pouring.
The sand is moist or green when it is compressed into the flask, and has to be dried in a kiln before pouring. Does sand really stay in the mould!? Two processes are responsible for the stability of the sand once it is compressed. (a) The dilatant nature. (b) The sand has 8-15% clay as binder and up to 5% of water contents.
A simple test is to compress a handful of sand and throw this about half a meter into the air, and if you can subsequently catch it as a whole, you might just have a good moulding sand. There are other important criteria to be matched: Permeability, refractoriness and a fine grain for surface smoothness. Fine quartz sand (or rather coarse silt) fulfills all conditions perfectly, if there is a clay contents between 8-15 wt%. Moulding with naturally bonded sand is easily one of the most fascianting techniques, because of its simplicity and its apparent defiance to obey the law of gravity. Once you have seen a flask -a metallic frame that has no bottom and no lid- being turned without the sand dropping out of it you will surely believe me.

Moulds, patterns & co

Best suited for sand casting are simple patterns without too many undercuts. Because the pattern needs to be withdrawn to create the mould cavity, the flask consists of two halves. To mould a pattern you first have to decide were the parting of the mould is. The parting is that line, where the mould cuts the cavity in two halves. It is evident that neither side can have undercuts, or you cannot withdraw your pattern.
First we create a false half, which holds the pattern in place: The pattern is buried up to the parting line of the pattern in sand. The sand is compacted and smoothed to produce a nice parting surface arounf the pattern. Then you powder this surface with a parting agent, to prevent the green sand sticking to the model or the parting surface. Put on the cope, put finely sieved sand on top of pattern and parting surface and compact it carefully with your hands. Put more sand into the cope and compress it with tampers and mallets. The whole flask is now turned, and the false half (drag) removed. It can be discarded and is shaken out. Put the drag back on, and repeat the steps previously described for the cope. After the drag has been made it is removed again, however, this time carefully, as to avoid damage to the mould cavity (You might want to lift the drag by pushing wedges into the four corners of the flask and thus to lever the drag slowly and in a controlled way from the cope. After that you tap the pattern and withdraw it carefully. Now you have produced the mould cavity. Downsprue, sprues and vents have to be cut into the sand, so the metal melt can reach the cavity. After drying the mould in the kiln it is ready for pouring. Cope and drag are reunited, clamped and metal is poured in. After casting shake the sand: your first cast!
What happens with more complicated patterns, e.g. sculptures? Can you cast them in sand? Yes, you can; if I did not manage to confuse you as yet, you may read further on the Stückform process here.

A lion aquamanile from Halberstadt, Germany

The lion aquamanile you can see here I modelled after the aquamanile from Halberstadt. It is to date one of the very few aquamaniles that is still in its original context (Mende 2008). Aquamaniles, somtimes also referred to asaquamanale, aquimanile, aquamnilia is composed of the the two latin words aqua, for waterr und manus, for hand. The term aquamanile was used differently in medieval times than today: it mostly referred to the receptables of the water in the form of dishes or bowl. In contrast the vessel for pouring water was called urceus, i.e. latin for ewer (Wolter-von dem Knesebeck 2008, 217). Aquamaniles were used for the ritual cleansing of the priest before the mass. Apart from this ecclesiastic use aquamaniles were also to be found in secular households of high social status where they were used before meals.

During the last month or so I modelled this 13th century aquamanile and cast it in bronze in my casting workshop using the lost wax technique. The model was made from wax and was prepared over a core of loam, just as it was described by the benedictine monk and artificer Theophilus Presbyter in his schedula diversarum artium in the chapter on the production of the cast incense burner. See here how the  article on the casting of the aquamanile  went in the reconstructed medieval loam mould.

To this end wax plates were applied to the loam core. This is also described by Theophilus. An alternative way to produce a wax layer of sufficient thickness would have been the repeated dipping into liquid wax. This however was not mentioned by Theophilus and therefore not used. After the wax plates had been applied the finer details, such as the mane or the eyes of the lion were shaped.

The aquamanile weighs 3,6 kg and holds 1.35 l of water. This reconstructed aquamanile may be seen in its original function at the events of the french re-enactment project Fief et chevalerie. Contact me if you would like to purchase this or any other aquamanile.

References:

Ursula Mende 2008.
Catalogue entry 52 in: Michael Brandt (Hrsg), Bild & Bestie. Hildesheimer Bronze der Stauferzeit. 378p. 2008.

Harald Wolter-von dem Knesebeck 2008.
Zur Inszenierung und Bedeutung von Aquamanilien, in: Michael Brandt (Hrsg), Bild & Bestie. Hildesheimer Bronze der Stauferzeit. 2008.

 

A very brief introduction to casting processes

The production and casting of metals is a very old trade and is known at least for the last 7000 years or so. Early evidence for mining can be found at Ai Bunar, Bulgaria and Rudna Glava, Serbia from 5000 BC onwards. Casting is a very efficient and cheap way to manufacture a desired good. This is especially true for metals. Casting processes can be discerned by the way a mould is produced. There are two possibilities: permanent moulds and lost moulds.
Lost moulds can be used only once. In order to retrieve the raw cast you have to destroy them. Permanent moulds can be used for multiple casts; they are usually made of two, three or more parts, to allow for the retrieval of the cast. Lost moulds require a pattern permanent moulds do not.

Moulding process should further be classified by the way the model or pattern is handled. There are lost models and permanent patterns. Generally patterns is the term for models used for sand-casting, whereas with the  lost-wax method it is often called a model. But as far as I can see, the English/American terminology is unfortunately rather ambiguous. This page is concerned primarily concerned with heritage casting processes, which is why modern processes are not discussed here, however I make use of modern terminology wherever feasible.

Lost moulds and lost models These are the most important lost mould processes with lost models:

• lost wax process
• traditional bell founding
• traditional gun founding

In the literature lost moulds are often said to be moulds of the lost wax or cire perdu process. This is incorrect as sand moulds are also lost moulds, as are the others mentioned above. In brief, the lost wax process uses positive wax model which is being invested in a moulding material, then heated, upon which the melting wax produces the mould cavity. This requires an individual model per cast. Early bells were made like this. The English/American terminology in the humanities coined the term direct and indirect lost wax process. This addresses the means of making the wax model, i.e. if each wax model made individually from scratch, or if  the wax model in itself was replicated by more efficient means, e.g. from a mould.

Some time after the 12th century the use of the lost wax method for bell founding was replaced by a more wax-saving process, which we nowadays call traditional bell founding process. This uses a strickle board to make the bell model from fine clay, which is used to make the two-part mould. Before casting the false dell (clay bell model) is removed. A similar technology came to bear in the production of cannon and, again, slightly modified also for the production of vessels such as cauldrons.

Lost moulds and permanent patterns

The single most important method mentioned her should be casting in sand moulds, which re-uses its permanent patterns. Sand moulds on the contrary possess permanent patterns which are being reused every time a sand-mould is made. Casting in sand moulds cannot be traces back very far. The earliest mentioning of this technique in our hemisphere is probably Al Jazari aorund AD 1200 . Susan La Niece published an excellent article putting forward the scarce evidence we have for early sand casting . In central Europe the earliest evidence is tzo found in written sources of the 16th century . Suggestions put forward by Egg that Gregor Löffler had introduced sand casting to gun founding are without any substance and reveal a simplistic view of foundry processes by that author.

Moulding loam – an ingenious material

In general, a moulding material should meet the following requirements:

• refractory
• no shrinkage
• gas-permeable
• fine-grained
• slight decay after casting (not relevant for older processes)

Since pure clay is too fat, i.e. it shows too much shrinkage in pure form, the clay must be made leaner. This is done with inorganic and organic materials, which is called temper or grog. Archaeologically proven and/or historically/ethnographically documented temper is e.g.: (quartz) Sand, crushed old forms, animal hair, horse manure and chaff. The clay acts as a binder for the inorganic, refractory ageing agents. Since these do not shrink during drying, the shrinkage of the moulding material can be adjusted via their proportion.

Organic tempers have a different function; they improve the plastic properties in the green state. Due to their fibrous shape, they reduce cracking during drying. They improve the gas permeability as they burn when firing the moulds. The moulding loam is/was produced by each foundryman himself/herself and can be produced with experience at any place where work is/was done. The raw materials are available everywhere.

Historical sources such as Theophilus Presbyter in the 12th century , Vanoccio Biringuccio  or Cellini  in the 16th century share their recipes with us. Also at Lazarus Ercker, the famous sampler and metallurgist of the 16th century, you can find some hints on how to use clay to produce refractory materials .

Dies, permanent moulds

You do not need a model, but you have to engrave a three-dimensional negative into the moulding material. You also want to be careful as to ensure that the cast can be retrieved from the mould halves. This seriously restricts the design of your object and at least for prehistoric times this moulding technique yielded far less complicated cast products than the lost wax process. They are also more labour intensive to manufacture. The advantage is the re-usability of these, and many objects can be cast without much effort once a mould is made. The materials for permanent moulds have to be refractory and hard-wearing, e.g. stone and metal.

References