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Mould Making, Ceramic Shell, Lost Wax Techniques

Materials, products, processes, suggestions and tips involving how sculpture is made.

Mould Making, Ceramic Shell, Lost Wax Techniques

Postby mantrid » Sun Jun 26, 2011 2:41 am

I was contacted by a guy who is reproducing a bronze aged artifact called the sun chariot.

http://en.wikipedia.org/wiki/Trundholm_sun_chariot

The bronze age people believed the sun was pulled across the sky by a horse and chariot.
He has asked me to produce the bronze horse part from his wax sculpture and he intends to do the rest (chariot, wheels disc etc). I thought I would share the process in pictures in the hope that the process will be useful to others and of course for advice form others who have experience of lost wax. I am almost on the sprueing so would be keen to hear how others would tackle this.

Here is the horse put together after receiving the parts. The legs were off for easier packing
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I scribed circles in the body that will reproduce the location of bronze rods used by the bronze aged caster to support the central core. I couldnt use this approach myself with the ceramic shell method Iam using as the preheating of the mould would probably melt the rods. I think the ancient peoply would have poured into cold clay moulds so no problem here
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This is the detail on the head that has to be reproduced. The grooves are very fine, less than 0.5mm in most places.
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I had to attach the bottom of the legs so it was important that the hooves where fairly square to line up with the axle that the horse will stand on.
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The horse ready for mould making just the pegs to add to the feet that will attach it to the axle.
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Decided to do two moulds, front and back. I did this so that I could have good access to the inside of the body so I could control the thickness of the wax better and thicken up bits afterwords thatI know will not allow the wax to settle on such as the pointed bits of the mould between the legs. I didnt take shots of every step but here are a few of the construction of the front mould. I decided to do it in 3 parts, on for each side and one between the legs
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The silicone rubber took the detail very well.
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Here are some pics of the rear mould stages done in the same way.
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I am concerned about the ceramic shell being tough enough to hold the fine detail without breaking or eroding away either during the pre heat or during the pour. So I cast some waxes of the side of the head to test with different consistencies and applications to see which shell will be the most resilient. I am using molochite 200, molochite 16-30 and molochite 15-80
http://www.specialplasters.co.uk/index.php?page=0&act=viewCat&catId=7
and primecote binder
http://www.specialplasters.co.uk/index.php?act=viewProd&productId=110

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Here is a quick cast of the horse just to see if the two halves would fit together and the finished moulds.
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Things I have to consider now
Do the bronze casting in two halves and weld together.
Put in a solid core and join the waxes together and bronze cast as one. piece. Preferable but I might not be able to fit it in my small kiln.
Sprueing, whats the best way?

More pics to follow.
mantrid
 
Posts: 45
Joined: Thu Jun 23, 2011 1:45 am

Re: Mould Making, Ceramic Shell, Lost Wax Techniques

Postby mantrid » Sun Jun 26, 2011 2:45 am

Ive been experimenting with wax to try and produce the pattern ready for application of the ceramic shell. The problem I had was that the detail is so fine that the surface tension of the hot wax stops it from getting into the fine detailing of the mould.
First problem is that the properties of the slushing wax that produces very good even hollow casts is very poor at getting into the detail. This I think is because it has a very narrow temp range from its liquid state to its gel state. What this means is that it sets easily on contact with the cooler mould forming an instant skin. The problen is that this happens before it gets ito the finedetailing.

My initial solution was to try the green wax that is designed to be painted on. Although this was better it still didnt get into all the cracks.

I tried painting the green wax on the detailed section before closing the mould. Then slushing with the pink wax. This worked better bet still not perfect. The problem is the detail is so fine that even when brushing bubbles can still get trapped in the tiny crevices.

I needed to reduce the surface tension so I tried swilling the mould with turpentine then pouring the wax. The idea beind it is that the terpentine would get into all the detailing very easily and when the wax was poured in it would disolve in the turpentine and fill the detailing. This worked! However, the turpentine made the wax very soft and one touch in the wrong place and the wax was smudged. Not ideal if you have to sprue or chase the wax. But would be ideal if you could avoid touching it in the wrong places and get the slurry on pretty quick. I did notice later that after a few days the wax did get harder, presumably as the turps evaporated out of the wax. I would use this method if I couldnt find a better one.

I finall settled on a very thin brushing of the detail with turps and allowing it to evaporate so it was just damping the surface. Then I poured in the green wax (just in the head section,which I found to be the best at penetrating into the detailing. I then used a bottle brush inserted into the mould and brushed the surface of the mould in the detailed area and agitated the wax, effectively brushing it into the detailing where the smaller amout of turps would aid its penetration. Ithen poured out the excess and used the slushing wax to give a nice smooth hollow pattern.

This was the best result so far. There are still some very tiny bubbles that I couldnt get out but I think I will proceed with this to the next stage and see how it turns out

If you are using Firefox browser press shift 'A' to get a better resolution

The boys cooling in the snow ready for chasing
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Rear mould, Yes Im a messy pourer. Doesnt help that the garage temp is sub zero.
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Close up showing the smooth finish and even thickness achieved with the slushing wax
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One of the heads. Note that the slushing wax has washed the green wax away and replaced it in the detailing. Not that it matters but I can resolve this by not leaving the slushing wax in as long the first time.
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Close up of some of the detailing showing the tiny bubbles. A solution to this may be to dab tiny drops of wax onto them creating a tiny lump of bronze in the final cast that can then be filed down level with the surface.
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mantrid
 
Posts: 45
Joined: Thu Jun 23, 2011 1:45 am

Re: Mould Making, Ceramic Shell, Lost Wax Techniques

Postby mantrid » Sun Jun 26, 2011 2:46 am

Sprued and ready for shell. Ive probably gone a bit over the top with it but better be safe than sorry. Just awaiting a delivery of plaster for the core. My current lot has picked up too much moisture.

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This one reminds me of the film alien
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mantrid
 
Posts: 45
Joined: Thu Jun 23, 2011 1:45 am

Re: Mould Making, Ceramic Shell, Lost Wax Techniques

Postby mantrid » Sun Jun 26, 2011 2:46 am

Ceramic shell stage. Its a slow process at the moment. Even indoors its taking all day to get each layer dry properly. Probably wont risk burning out until early next week. This pink wax I suspect will expand quite a bit. Dont know if it can flow out fast enough to releive the pressure so I want the shell as hard as possible before I try.

This one has been drying all day and its still damp in the reccesses as you can tell from the yellow colour. Orange means dry.
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The little bumps on th body are the ends if the pins supporting the core, which was about equal quantities of casting plaster, perlite, sand and molochite flour.
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mantrid
 
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Re: Mould Making, Ceramic Shell, Lost Wax Techniques

Postby mantrid » Sun Jun 26, 2011 2:47 am

I burned out the shells. They came out nice without any cracking and no bits fell out so I think the shells are nice and stable.

I didnt want to chance any cracking from the pressure and so drilled some small pressure relief holes in non critical places around the shell. I havent done this since my woman in the surf bronze that had many flat surfaces and sharp edges. You can see the location of these as yellow patches where I filled them and the surrounding few cms with thick slurry after the burnout.

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Hope to pour the bronze tomorrow if my propane tanks dont freeze up. May see if I can get an electric blanket or something
mantrid
 
Posts: 45
Joined: Thu Jun 23, 2011 1:45 am

Re: Mould Making, Ceramic Shell, Lost Wax Techniques

Postby mantrid » Sun Jun 26, 2011 2:48 am

Well did the front half of the bronze yesterday. Couldnt do the back half as the propane tanks were so cold they werent giving out enough gas to melt quick enough. I think I would have run out before it got hot enough.

The result looked promising until I removed all the shell and a few defects were seen. I may have got away with it but one was in a critical region. I'll probably get a new wax set up and do this again. In the meantime theres no harm in trying to put a weld in that critical region and try and chase the pattern. I can grind some chasing tools from old drill bits. I could do i with a punch with a triangular head and another thin gouge type chisel edge.

I may also consider saving the body and casting just the head then weld the head on. I coud do a few heads at the same time as backup.

Small hole near to where the sprue entered
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Some small holes in the eye, towards the snout and in the cheek area.
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The largest of the holes. but not affecting too much of the detail. Chasing back the pattern wont be the problem but damaging the surrounding area with the TIG maybe. Anyone with welding experience who can advise on tackling this one?
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mantrid
 
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Re: Mould Making, Ceramic Shell, Lost Wax Techniques

Postby mantrid » Sun Jun 26, 2011 2:51 am

I decided to recast the head as the welding repairs would have been too tricky in the area od the detailing.
The finished horse.
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mantrid
 
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Re: Mould Making, Ceramic Shell, Lost Wax Techniques

Postby bsilverman » Sun Aug 21, 2011 10:08 am

I would like to see the missing photos. In particular how you make the inside of the legs. I am trying to make a similar mold and that is the area that confuses me the most. This is my first time to make a mold. Not going to do lost wax, just plaster of paris pour. Thanks in advance if you can show the pics.
bsilverman
 
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Re: Mould Making, Ceramic Shell, Lost Wax Techniques

Postby Giotto » Sun Aug 21, 2011 6:01 pm

beautiful work Mantrid....

G
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