Vikings for all occasions, no bronze too hot
Comments on this site should be sent to Roger Barry
Keep an eye on the state of the bronze (too much heat for too long will cause the loss of tin from the bronze and make casting more difficult). Once the bronze is molten, top up the crucible to ease slag removal and allow this to melt.
De-gas bronze with green oak poles? (Calcium carbonate is a known de-gassing flux).
Scrape off major slag and recover with hot charcoal.
Reposition crucible and flask if necessary to ease their removal from the fire. The crucible should be positioned so that when grabbed by tongs the correct, pouring side is at the lowest point when tipped.
Heat tongs in the side of the fire to prevent conduction cooling of the flask. Get the metal HOT (a nice yellow glow and very liquid melt indicate a good pouring temperature).
The next set of operations must be done in quick succession as the flask and crucible cool extremely quickly.
· Push aside charcoal and scrape off any left in the top of the crucible.
· Remove flask and crucible
· Pour by missing with top layer of bronze then fill the flask
· Top up quickly if necessary or practice pouring!
· Tap gently to help remove bubbles and get the metal into all the features.
· Leave flask to air cool
Break open flask carefully.
Keep the bits of the flask to use as grog, or display.
The Ghanaians cast bronze/brass with a built-in crucible in each flask! (Minimum gas absorption, no contamination and bubbles can rise out over a longer time). Need to know the volume of bronze accurately to ensure proper filling of the mould.
Two piece stone moulds could be reused again and again, gas escapes between the two mating surfaces and they are capable of a good surface finish. Flat backed castings would be easy to make, with carving on only one side. Moulds could be clamped by rope or supported by sand. Stones suitable for casting include: - sandstone, steatite, chlorite- talk, schist and greenstone.
If opened before too much shrinkage occurs these could be the best multi-use moulds available.
This information has been acquired during shows over 4 years at West Stow Anglian village and we are now obtaining a successful cast of about 1 in every 3-4 attempts!
Many thanks go to Russell Scott for his experience and enthusiasm in sharing his knowledge.
To attempt bronze casting you need: -
· Wax (proper casting wax is good, candle wax a bit brittle, bees wax a bit soft, perhaps try a mix? note. 1:1 is still too brittle),
· Fine clay (any sort should work, or puddle local stuff i.e. let a very wet mix settle out and take the top layer of fine clay)
· Course clay (Raku is very resilient, but it would be good to try local clays)
· Sand and grog (old broken flasks etc should be kept for this purpose) to temper the flask clay.
· Grass fed horse dung (this should keep if dry [watch out if it goes off])
· Round section dry grass, reject if crushed (you need the central hole intact).
· Darkish area to work in (non-flammable roof and floor!) with adequate space to move around the fire and no trip hazards.
· Forge with a pair of good bellows (a powered fan is a cheat, use a muffle furnace!!!!)
· 2 or preferably 3 people (one should be a good bellows operator),
· A crucible (should try one of sand tempered local clay),
· Fire bricks or clay dam to contain the fire.
· Long handled tongs sized to hold the crucible and long handled tongs to fit the flasks
· Aprons for both pourer and holder and gloves for both, safety specs and boots optional.
· Small lumps of bronze (too large lumps can be smashed easily if hot enough to glow, watch out for the red hot bits that fly everywhere if you hit too hard).
· A supply of good quality charcoal (yes this stuff is very variable, you don't want the stuff that showers you with sparks when under the blast).
· Charcoal shovel and rake (a normal fire shovel with a lengthened handle is ideal).
· Bucket of water (used as water supply for clay work, cooling for tools and burn relief!)
· Drink (its hot work!).
· Assorted tools for wax carving and fine blade for welding wax.
Add a pour cone and extra space in the base to reduce failure due to trapped dirt. Add any major sprues and gates if the wax model is complicated or could trap air. You should also make far too many waxes. You never have enough. Place multiple waxes in a single flask to increase yield per pour.
The making of the flask should be completed in one continuous operation, otherwise cracks form, between the layers and the cast object will be difficult to release.
Apply a very thin layer of fine clay, by hand, to the wax, pushing the clay into all the features. Using a mix of 50/50 clay and grass fed horse dung as flask material, make the flask a cylindrical shape with flat sides to ease holding in tongs, keep the flask's neck thick to keep the heat. Add run-off holes to the flask, which allow thin needles of flash to form in all directions and ensure that the metal runs to all parts of the cast and air locks don't occur. Medium fine grass straws should be used.
The flask should be at least 3cu" in volume to reduce the speed of heat loss. Flasks should have a reservoir of the same volume as the wax as a pouring cone; this allows settling and shrinkage to occur. The excess will be reused anyway.
To reduce the risk of flask fracture, the flask walls should ideally be of consistent thickness to prevent uneven rate of change of temperature during pour/cooling.
Fire flask and burn out wax and organic matter. Clean up if necessary, the neck sometimes needs opening up (initial heating should be slow to reduce cracking).
Always check the contents of your crucibles; dump if unsure. Load crucible with known good bronze (possibly top up with tin to rebalance the alloy if using old bronze from pervious melts, the colour of the last casting will tell you if the metal is becoming copper rich [a noticeably redder colour]).
After running the fire assess the extent of the glowing portion. Build a wall to restrict the fire to this size, the top of the wall should be as high as the top of the crucible.
Place fired flask upside down next to the crucible and cover both with burning charcoal.
Fire management is important, a hollow fire will produce glass on the crucible and this runs down and can glue the crucible to the base! It also slows melting.
Heat until green is seen on charcoal or in the flame (the bronze is starting to melt), a steady medium blast is better than going at full blast for the melting (save your energy for the pre-pour heating).
Ensure blast is even around crucible & flask to ensure even heat distribution. Ideal is for the Tulare to terminate below the crucible base. There is some thought that a tangential blast (as used in commercial furnaces) might improve heat distribution. (More research required [is this with gas-powered furnaces?]).