Kynnabaria

Casting

Casting as a metalworking technique started in prehistory and still thrives today- in some cases using nearly identical techniques. Such easy and dramatic reshaping of metal can also be a very satisfying hobby. This article will discuss casting in general and will explain the basics of one casting method normal people can use.

Casting technology- and there's a LOT of different methods and techniques- can generally be divided into 1) how the mold is made and 2) how the liquid metal gets into the mold. The second question is easier to answer, but in fact there are several choices available even for a kitchen foundry, e.g. centrifugal casting, injection with steam pressure, or gravity feed. There is substantial evidence to suggest gravity was available and employed in Viking Age casting. For beginners, today's gravity is still the easiest and most readily available contraption available to get metal into the mold. Some things never change.

There are two mold-making techniques, broadly speaking. The first option is to start with mold material and directly carve into it a negative image of the piece to be cast. Viking Age soapstone molds like this have been found- one of the more famous examples is a rectangular piece with cavities that produce crosses and Thor's hammers. The second option is to start with an original piece and wrap a mold around it. Viking Age parts from clay molds like this have been found, and even reconstructed to show the tortoise-shaped brooches they produced. This second technique has attractive features for beginners.

"Sand" casting, a subset of the latter method, is what I'll be discussing. The quotes are there because the mold media doesn't have to be sand. The casting technique I'll discuss uses Delft(tm) clay instead. I cannot document sand casting (or casting into Delft(tm) clay) as a Viking method; furthermore, I have never seen the tell-tale frames one uses for sand casting in Viking Age finds. Furthermore, the available evidence makes a claim for sand casting in the Viking Age fairly indefensible. On the other hand, once the results are cleaned up one cast part looks much like another, regardless of technique. The main limitation of this technique is that undercuts are impossible to reproduce, which is a problem shared with the period Viking techniques.

Required tools and materials:

• Delft(tm) clay
• a small hammer
• a straight edge (like a ruler)
• a two part frame
• safety glasses and heavy gloves
• some metal
• a crucible
• tongs
• a heat source

Duplicating original pieces by clay casting is very simple. Please read step 11 first, then start at the beginning and read the whole thing again:

1. Remind yourself how dangerous molten metal can be. Think of lava pouring over a jungle village.
2. Get an original piece to be duplicated.
3. Get a two section, pull-apart frame (a cope and drag)
4. Pack the lower section with clay and "strike it off"
5. Press about half the original piece into the packed clay
6. Lightly Talc up the surface of the original that sticks out as well as the surface of the clay
7. Put the other frame section on top of the first, and pack it full of clay
8. Pull the frame sections apart and remove the original. At this point you should have a three dimensional negative made up of dents in the two clay-filled frame sections; it should look a lot like the original piece.
9. Cut a sprue to let the dangerously hot molten metal in and one vent or more to let trapped air out.
10. Melt some metal without getting any on yourself. Pour molten metal into the sprue without getting any on yourself. Did I mention molten metal can cause you harm?
11. Let the hot, extremely dangerous, lava-like molten metal COOL DOWN before you pull the frame sections apart to view your completed masterpiece (did you read this step first?)

2.  You're unlikely to get a lot of personal assistance gathering an original piece to duplicate.

3.  The frame can be purchased- try Rio Grande on-line at http://www.riogrande.com. I made my own from cut-down 1x4 pine lumber, #6 drywall screws and a couple nails; it works just fine. In fact, you could nail or glue the frame together; I figured since I'd be hammering on it I would use screws. Here's a sketch of the frame I made:

4.  To pack clay into either frame section, I start packing clay in with hand pressure and end up hammering it down with an auto body hammer. Fragile originals might need a gentler touch. When the clay is packed, one "strikes it off" to make it level. Any stiff, straight-edged device will strike off the clay: a ruler, a piece of wood, etc.

5.  Pushing the original piece into the first packed frame section can be difficult. If the part refuses to sink into the clay, carve a small depression to fit the piece, and try to get about half of the original pressed into the first frame half.

6.  Talc up the exposed surfaces. The talc creates a split line by discouraging the casting clay from sticking to itself. The split line is where the clay in the frame halves will separate. I use Johnson Baby powder and a cheapo brush from the hardware store.

7.  Put the second frame half on top of the first. Note the nails guiding the frame halves together. I also marked matching faces on the frame halves to make sure the two parts go together the same way every time. Put clay in the top half of the frame. Pack it down, hammer it down tight, and strike it off.

8.  Gently pull the frame sections apart. Remove the original piece without majorly disturbing the mold cavity it made. You should have an indentation in both sides of the mold, something like this:

9.  Cut a sprue or sprues. Put the incoming sprue into the thickest part of the piece; this helps keep the metal molten as it pours into the mold cavity. For very broad parts it may be an advantage to use more than one sprue. Remember, the metal has to move through it, so the sprue should be a decent size- I generally use ~ quarter inch sprue for pewter and larger for bronze. Poke the nail through from the inside of the mold (yes, this messes up your mold somewhat) to the outside. Centering on the sprue hole you just poked through, cut an inverted cone over the sprue. This is the target for pouring molten metal..

Cut a vent or vents. These can be smaller since only air has to move through them. Use the same nail to poke the holes through, but scratch lines leading from the mold cavity to them. Make sure clay does not pile up at either end of the scratch. For a look at vent holes, see the closeup of the casting still in the clay.

10.  Melt some metal. Don't get any on you. Touching molten metal hurts a lot and can permanently injure you.. For heat I used MAPP(tm) gas available from the hardware store and bought and add-on flexible hose/hand valve/large tip package. This gets combination gets hot enough to melt bronze- barely. It also allows me to hold the canister upright while I work, which appears to be a big advantage in terms of keeping the flame lit. A lot of these kits come with a fairly convenient flint striker. Pour the molten metal into the sprue. Don't get any on you. Did I mention very hot metal can hurt you? Safety glasses and gloves are a must!

11.  Let the metal cool down until it's solid. It's tempting to throw some water on the sprue button to cool it faster. Unfortunately, if the metal is still somewhat liquid the water droplets can push down into it before they're boiled off. As the droplets turn to steam, they'll launch burning hot molten metal out at you. This appears to hurt. Don't do this; let the metal cool down and solidify.

Once the metal has cooled off, pull the frame halves apart and knock the clay off your new masterpiece! You're done! Congratulations. Now you get to cut off the button and file, grind, sand and finish the piece.
 

Here's some melting points for commonly available metals: