Sand Casting with Bronze

Bronze is basically the same as the pewter mentioned in the Easy start with a few added dangers.

Disclaimer I take no responsibility for you doing yourself or anyone / thing else harm if you have a go at this yourself!
you are strongly recommend to have safety clothing, fire extinguishers, water and any other things you may like to add for safety.
ONLY use water on people / animals that are in trouble! NEVER pour water on molten metal or even allow any water to get near to molten metal as it can explode violently!
If at all possible allow any molten metal to cool naturally even if it does start to melt a hole in your drive let it ruin the drive rather than risk cooling it too quick and causing a shower of molten metal to go every where! Better still, of course, keep the stuff where it's meant to be.
NB. If you go to Kentwell Hall in Longmelford take a look at the roof of the foundry. Some water once got into the relatively cool molten lead, the resultant explosion has covered the roof (and founder at the time) in specs of lead.

Equipment

Bronze

True bronze is a mix of Copper and Tin. Both metals are relatively soft on their own but when mixed together become hard. The more tin you add to copper the harder and more brittle the bronze becomes. In general if the mix has much more than 23% tin it becomes too brittle to be useful. Bells use around 20% tin so they are really very hard so ring well. Most other bronze items use around 10% tin known normally as gun metal.
You can get scrap bronze from various sources, bearings, old bells etc. but note that not all that glistens is bronze. Most 'bronze' items are at best impure and at worst poor quality brass masquerading as bronze.
You can melt most of these just as easily as each other however be very careful! They may contain substances that when heated evaporate into the atmosphere and your lungs. One of the commonest ones is Zinc (brass is copper and Zinc) which I believe to be harmful if breathed in. Always do your melts in a well ventilated area and if using scrap be prepared to have to abandon the melt in favor of cooling the substance to avoid excessive exposure to harmful gas’s. Better still, see the suppliers section for a list of places you can get good bronze from.
The difference in casting quality between using scrap metal and pucker bronze is phenomenal. Always go for the good stuff as you'll be much happier with the results.

Molding Sand

Mansfield red sand. Don't need to say much more really. You 'can' make your own molding sand but it's hardly worth the effort. Get yourself either Mansfield red sand for mould making or if you can justify the additional cost get hold of some oil bonded molding sand which can produce a better result.

Molding Box (cope + drag)

This is the item we will be using to make our mould in. In short it is 2 boxes with no top or bottom that can be keyed together. They should be able to be taken apart and put back together again in exactly the same position repeatedly. You can get metal molding boxes or wooden ones. I use wooden ones that I built myself out of 3 x 1 planed timber. Metal ones are heavy and cost to buy, wooden ones are generally lighter, cheap to make but don't last long if you happen to get molten bronze on them which is bound to happen eventually.

Crucible

The crucible is a cup like item, made from a mix of clay and a refractory material, that will contain your metal whilst it melts. If you really know your pottery it is possible to make a crucible but I would strongly advise buying one.
Bronze requires temperatures above 1000'c (yes, OVER one thousand degrees centigrade!) to melt and nominally in excess of 1100'c to be ok for casting with. The crucible has to withstand these temperatures and then some. The crucible in an uncontrolled system could easily get to 1500'c and you don't want it breaking and spilling molten metal all over the place at just the wrong time.

Furnace

A furnace is little more than a contained heat source. There are a number of furnace designs using things like electrical induction and molecular excitement (microwaves) however most furnaces for metal casting rely on a fuel and air mix. Get the mix correct and plentiful quantities of each in a confined space and you'll get lots of heat.
I use a metal bucket lined with fire cement, to stop the heat conducting to the bucket itself. Into this I place blacksmiths coal nuts or charcoal when I can afford it and force in air via a copper tube connected to an air bed pump. See the home foundry section for more details.

Method

Notice that the method is almost exactly the same as for pewter. However, the equipment is different so do read on.

Get a master

Wooden bell master

We start off with a master. For this one I'm making a small bell known as a cup bell designed to be struck with a 'hammer' (small wooden or rubber thing) so I don't require a clapper to be fitted.
The master is made from wood purely as I can turn wood to the desired shape. You can use any solid object, but take care about the shape as some are more difficult to make a mold from than others. This master has what looks like a short handle on the top (not shown) which will become the sprue into which we pour the molten bronze and finally become the handle.

Later correction: (Jan 2006) Following a visit to the Whitechapel bell foundry I notice that they pour into the bottom of a small bell rather than the top as I have done here. At the time of writing I haven't tried pouring into the bottom but will be experimenting with this method.

Later correction: (May 2006) Experimentation has shown that pouring into the bottom of the bell (mouth) creates more successful results. It allows the air to escape freely through the sprue at the top however a good head of bronze is required.

Make a mould

For more details on mould making check out the mould making section. Here we just cover the basics.

13 First we put the master on a flat board and place the cope half (top) of the molding box over it and dust it with parting powder.

Start to fill this half with sand and ram it in using a rammer. Before it is full put a sprue stick on top of the master. A sprue stick is a taperd piece of wood which when removed will create a hole (sprue) through which we will be filling the mould with molten metal. Then fill the cope to the very top with molding sand. This will need to be stickled flat.

14 Now we invert the cope and place on the drag. The drag will become the bottom when we invert the entire thing again later. Here we see the underside of the master and the molding sand surrounding it. This is coated in parting powder before being filled with molding sand, rammed up and strickled off.

Turn the entire box over once again, remove the sprue stick and clean up it's entrance hole. Carefully pull off the cope from the drag and place it to one side. Remove the master from the mould, it may be in either the cope or the drag but it becomes obvious as it's the only lump of wood in there. Clean the mould as / if required and place the cope back on the drag. We now have our completed mould ready to accept the molten metal.

Melt some metal

Fire in the furnace Now we take the furnace (details in home foundry) and get a fire going.
Bronze melt starting Into the furnace we place the crucible and into that the bronze.
here I'm using reclaimed bronze (a previously cast bell that wasn't do good) which doesn’t 100% fit into the crucible initially. It is however stable and can remain with a little protruding over the top until it melts enough to sink in.
Crucible with lid on in the furnace When all the material is within the crucible we put on a lid to conserve heat, reduce the amount of impurities (coal, charcoal, fag ends etc) that can get in and it also reduces the amount of tin that may evaporate off.
Molten bronze in the crucible When all the material is molten...
Fully molten bronze in the crucible We turn the heat up a little so that it is hot to the point that it won't solidify when we move the crucible from the furnace to the nearby mould.
Note the EXTREEM heat required for this operation, no old electric stove for this one! Due to the design the heat nearby is limited but put your bare hand anyplace above it for a short time and you'll lose the hairs followed by the whole hand very quickly!
The bronze is at over 1100'c for that the crucible will be hotter and the fire hotter still. I don't know the temperatures in the fire but the coal can melt before it's burnt up and poking the fire with a copper tube can only be done briefly (30seconds or so) before that starts to melt. This is why I started this page with all the safety warnings.
I'm no where near as safety conscious as I should be, the local A+E will testify to that, but I wouldn't 'mess around' with this stuff.

Next step is to remove the crucible from the furnace and CAREFULLY pour it into our mould. These are the most hazardous parts of the operation.
The crucible can slip or crack, the operator can slip, the tongs can break, the mould can explode or leak to mention but a few of the possible things that can go wrong. All the time the operator is VERY close to the molten bronze and ANY of the mentioned problems can mean they are covered in molten bronze. If this were to happen I can only suggest doing what you think if best at the time. Molten bronze will destroy skin and tissue on contact and, I would guess, bone in a short time! Stout leather boots, apron, chaps, gloves goggles will give you a few seconds to react and hopefully do the right thing. What that is depends on the situation.

Pour the molten bronze into the mould

Initial lifting of crucible from the furnace Firstly we need to get the crucible out of the furnace. This is done by using tongs to carefully raise the crucible from the fire a bit at a time each time taking a better / lower grip on it. It's important to have a firm and secure grip without putting too much pressure on the crucible.
Crucible lifted from the furnace Clamping too hard at this stage could cause the crucible to break. As the operator has to be too close (within the same county) at this stage any problems here could be disastrous.

The crucible is transported the short distance to the mould. This should involve no more than one or two steps. The mould should be close to reduce the time the crucible full of molten metal is in the operators hands but far enough away as to not represent a trip hazard should a problem occur at the furnace.

Pouring the bronze into the mould The contents are then carefully poured into the mould...
Pour completed, empty crucible next to cope and drag filled with bell metal and left to cool. The cooling for a bell of this size (1kg) to a temperature that is just acceptable takes around an hour but is better off left for a couple of hours and then still handled with care.

Video clip of melt and pour (4.5Meg wmf file) Containing clips from 3 different pours. The first part is late in the evening giving you some idea of the heat generated.

Take the finished object from mould

Initial knocking out of the bell from the mould When all has cooled sufficiently we begin the shakeout. traditionally the mould is literally shaken out so that everything in the box falls out and the finished item is extracted from the pile of sand. I however normally dig out the good sand from the mould as far as possible and put it to one side carefully. With the moulds being quite small the percentage of burnt sand (that which in closest to the finished item) is high and I don't want too much of this mixed in with the good sand.
Completed cast bell unfinished Finally we get to the cast item. It doesn’t look too much like a quality bell at this stage. It is covered in a thin layer of burnt sand, still has the sprue waste attached and is fairly rough where it has taken the finish of the sand.

NB:- Since this site was written improvements have been made to the process such that finishing is normally only cutting off the sprue and a little fine polishing.

Finishing touches

Part cleaned bell The sprue waste is trimmed down and the bell spun up in a lathe to clean up the outside. Traditionally the lathe work is done by hand more like wood turning than metal turning. This one however was done using metal turning methods as I don't yet have the appropriate tools to do it by hand. You’ll notice the rim has not yet been turned at this stage. This amount of cleaning up should not really be required as I have found since making this bell.
Completed bell Finally the completed bell. Once the bell has been polished it is a good idea to avoid excessive handling as finger prints show quite clearly as I feel is evident here. At this stage you can modify the spigot on top to take a leather strap, wooden handle or for mounting in any number of ways. You can insert a clapper if required by drilling and tapping the inside or just leave it, like this one, to be struck by a hand held 'hammer'. By their nature bells are brittle so don't go hitting it with a 2lb club hammer unless you are trying to disassemble it for remolding.

Conclusion

The principles are the same for bronze as they are for pewter or even lead. The higher temperatures do however require additional care and more robust equipment. It is important to ensure that every item in the process can cope with the temperatures it will be subjected to and then some more for security.
Essentially it is within the bounds of reason that this art can be practiced by the amateur with access to a private outdoor area.