View Full Version : Help? Best way to mold fired clay to bronze
06-05-2006, 01:08 PM
Hi. I would appreciate any advice. I am very knowledgeable regarding making silicone molds, usually for small wax figures. But I have sculpted several "Life Sculptures" from a red clay and had them fired. They stand approximately 18" tall and are pretty heavy.
I would like to take one of these sculptures and make a bronze. I would like to have the option of making several bronzes of this same figure. If possible, I would prefer to keep the original clay sculpture as well.
My thoughts are, I could pour a silicone mold and then a wax copy, and then give the wax to the foundry - which in fact, would save on foundry costs. The only problems are that silicone molding material for this size piece is quite expensive and heavy.
I was under the impression there was a more common and easier approach to molding this type of figure of which I am not sure. I think it involves building a plaster mold around the figure - to shape rather than a box - and then pouring silicone inside. Or perhaps it was to pour a plaster mold with some sort of dividers to create parting lines. Either way, I am not sure and would appreciate any advice on the easiest and/or the most cost effective way to turn my fired clay into bronze with the option of making duplicates in the future. I also would prefer to give the foundry a wax copy since it saves on foundry costs, but am open to any ideas.
06-05-2006, 05:14 PM
When using flexible mold material on larger sculptures, one uses a fairly thin layer that is painted or glopped on, not poured. This thin layer is then encapsulated in a larger rigid mold, usually plaster, which is also put on by hand in a shell shape, as opposed to any "pouring". Between the initial thin layer of flexible material and the outer layer, a filler material like rubber shavings is mixed in with the mold material to fill in undercuts. It becomes part of the flexible mold, so that the outer mold can be simpler with less pieces. If you make the 2 part mold well, many wax positive copies can be made. This is standard mold-making stuff. I've probably read 50 different descriptions of it. Try any number of library books or the educational videos that SmoothOn makes: http://www.smooth-on.com/howto.htm
06-05-2006, 06:35 PM
When using flexible mold material on larger sculptures, one uses a fairly thin layer that is painted or glopped on, not poured. This thin layer is then encapsulated in a larger rigid mold, usually plaster, which is also put on by hand in a shell shape, as opposed to any "pouring". Between the initial thin layer of flexible material and the outer layer, a filler material like rubber shavings is mixed in with the mold material to fill in undercuts. It becomes part of the flexible mold,If you use an already thixotropic silicome like Quantum QM140 you dont NEED to add anything, it's already like peanut butter and you can layer on over an undercut as heavy as you wish.
The only problems are that silicone molding material for this size piece is quite expensive and heavy.
Silicone, heavy? for a mold of something only 18" high? For that size at best you are talking about a one gallon sized kit - ten pounds worth- about $118 delivered to your front door. If 10 pounds is heavy to you, you sure won't like the plaster shell or the bronze because they will be alot heavier than that. If $118 for a mold is expensive to you, you might want to rethink this because the cost of this sort of project only barely BEGINS with the rubber and goes up, WAY up from there for shipping the wax, shipping the bronze, foundry casting the bronze, patina, mounting, advertising/selling.
Sending the wax to them saves SOME money, but remember your TIME plus the cost of the wax, plus shipping has to be considered v/s letting the foundry cast the wax for you along with a bunch of other waxes they will be doing that day too.
06-05-2006, 07:59 PM
I guess it figures my method would be outdated, since most of my education comes from older library books and self-experimentation. When I messed around with molds, I only used really cheap stuff - namely latex - because I couldn't afford anything. What you said about the cost became obvious fast, not to mention all the labor involved after I had already completed creating the sculpture's form. It basically drove me to wood carving, then to steel, in search of something better suited to me. When I think of all those steps and all those materials, it just increases my esteem for good ol' cheap steel and a one-step fabrication/creation process.
06-05-2006, 10:04 PM
Bronze is so damn expensive these days, just the raw metal is about $4 a pound, labor isnt cheap, neither is fuel, insurance, materials, so foundries have to charge enough. I was quoted $900 or so for a sculpture about 21" tall with me doing the wax and shipping it to Thailand.
A Mexican foundry quoted about $450, again with my shipping the wax to Mexico and the 60# or so bronze back and it's not cheap to ship internationally.
I used to get 6-8" tall figures cast in Joseph Oregon for about $125 but that was 15 years ago.
Ive come to the conclusion of late that the only way to go is to learn how to cast your OWN bronze, espcially since I would otherwise be doing 75% of the work ANYWAY, the only process I wasn't doing was the actual metal pour and gate grinding/chasing, I even did the patina and mounts myself.
So if I can set up a small studio furnace etc I can give it a try, there's also aluminum as well as lead-free tin alloys that are practical to cast at home without a lot of equipment or heat.
this may be a lot more involved than you would ever want to go of course.
06-06-2006, 12:53 AM
I've never liked the idea of working in more precious and expensive materials. I like the idea of cheap stuff that doesn't add any pressure to the constant screwing up necessary to the learning process.
I guess I'll find out more about this stuff, as I've decided to take a foundry class this semester at the U, with extensive access to all the other sculpture facilities included. At this point, I could take the whole casting/foundry thing or leave it. I don't see any inherent limitations to my current steel/gas welding process other than my own imagination and gumption. I think I need to see something unique to the casting process itself before I'll become really interested - something besides going through a lot of trouble to accurately replicate something made out of clay.
06-06-2006, 01:37 AM
Landseer - Yes, I do find a silicone mold poured into a box around an 18" figure with varying degrees of width (up to 9") to be heavy - at least heavier than it needs to be. Not to mention the weight of the piece alone. That is when pouring the mold - thus why I was looking for another way, such as layering. The way that I am used to pouring molds is for small figurines (with box around piece). Pouring this particular piece in that way would require more than the 1 gallon kit and the next size up costs $500. While I have some leftover from a previous kit, I don't want to be wasteful with such precious material. Regarding the foundry, I already have one and they are local and offering me very good pricing, as well as an opportunity to work with them and learn. I just need to deliver the wax, which I can do in the back of my truck. So, no shipping involved. I'm actually really surprised that people are going overseas. Mind you, it took several quotes to find this guy and negotiating to do the work with them helps me to learn while bringing down the cost. Perhaps someone in your area will do the same.
So, back to the mold. Thank you all for your advice. Questions: I have never painted the silicone onto the piece. The brands I use (Visil and Silpak) take a minimum of 45 minutes to cure, and a maximum of overnight (no pressure pot). I can't imagine that these layers will cure quick enough without just slowly dripping down. Do I just need to take a leap of faith? Also, what do you do about parting the mold? Should I just cut through the plaster? I've heard of people putting in some sort of dividers/ splitters, but I am not sure how that is done. I also have heard of putting a "lip" around where the parting line is to be joined with a certain type of screw for better securement later when pouring the wax. I know I'm not very clear (which is why I need you!) but if you have any info on these latter subjects I would appreciate it. Also, the silicone heats as it cures and I am wondering if it is going to bust/ crack the fired piece. Perhaps I should try this thixotropic silicone you mentioned rather than using my "leftovers"...???
I'll check on the "smooth on" how to. Thanks for the advice.
06-06-2006, 02:07 AM
Hey - That smooth-on site is great. I think I have learned that I definitely need to use a different type of silicone molding material. I'm still confused about the parting lines - the visuals weren't too good on that part!
06-06-2006, 02:36 AM
Landseer - Yes, I do find a silicone mold poured into a box around an 18" figure with varying degrees of width (up to 9") to be heavy - at least heavier than it needs to be. Not to mention the weight of the piece alone. That is when pouring the mold - thus why I was looking for another way, such as layering. The way that I am used to pouring molds is for small figurines (with box around piece). Pouring this particular piece in that way would require more than the 1 gallon kit and the next size up costs $500. I don't know anyone who pours solid rubber molds like that, at least not of anything large- it takes WAY more rubber than you need to fill such a space so most is wasted. It also makes the thing too stiff- like a car tire almost.
That is why you use THIXOTROPIC silicone, so it goes on with a brush and spatchula and doesnt run off vertical surfaces.
The rubber only NEEDS to be about 1/4" thick even a little less, if you have seen the pictures of my large pieces, most of them don't even use a gallon of rubber. My dragon panel 38" wide x 15" high will take about a gallon as will my large square lion also shown here. That's about a 10# kit and that's the size I buy, if I need more I buy TWO kits to be sure rather than the next size up which is the 5 gallon pail.
I figured the hydrocal cast of the dragon panel will be 65#, the rubber 10#, the rubber's weight whether it's 5# or 10# is MINOR compared to the 65# of the cast it will make.
I just need to deliver the wax, which I can do in the back of my truck. So, no shipping involved. Ok, but at $3 a gallon for gas plus your time that is still a cost, less than shipping to Taiwan mind you but if the bronze costs less in Taiwan or Mexico to cast than the USA the savings could more than compensate for the $100 shipping it might run, still, if you plan to sell the bronzes and you figure the typical 3x foundry costs and you figured out you paid $750 for the bronze, are you confident you will be able to sell that for $2,250 ?
One of the sculptures I got the estimate on- $900 in Thailand, was quoted $1500 here in the states by one place, could I ever hope to take even that $900 price x 3 and sell it for $2700? let alone $1500 x 3?? NO WAY!
Ditto for the small model I was quoted $150 to cast, there's NO way anyone is ever going to pay $450 for that little plaque.
I'm actually really surprised that people are going overseas. Mind you, it took several quotes to find this guy and negotiating to do the work with them helps me to learn while bringing down the cost. Perhaps someone in your area will do the same. I'm NOT surprised when people over there will work all day for $10 . One of the foundries has a LIFE SIZED bronze horse in their line for sale, it was something like $3750 us dollars, here it would be ten times that, the catch is shipping- container might cost you $2000 via boat but still way cheaper than similar items here.
In any case, no matter how much you negotiation the raw metal STILL costs $4 a pound and the bronze will never cost less than the materials. Good that you have a local option, I DONT, my local option was twice what the Mexican foundry quoted and even that was too high for me to triple and ever hope to resell the cast in my lifetime.
So, back to the mold. Thank you all for your advice. Questions: I have never painted the silicone onto the piece. The brands I use (Visil and Silpak) take a minimum of 45 minutes to cure, and a maximum of overnight (no pressure pot). I can't imagine that these layers will cure quick enough without just slowly dripping down. Do I just need to take a leap of faith? Never heard of those, I use QUantum Silicones QM 140 with the Blue Thixio catalist, it stays workable for at least one hour in a peanut butter like consistency that will not run or sag on vertical surfaces. I like to do it in about 3-5 applications an hour apart. It cures in about 8 hours but suggested it stay 24. I havent had a bit of problem with it- vertical, two halves keyed, thin, thick, 2 applications, 6 applications.
Also, what do you do about parting the mold? Should I just cut through the plaster? I've heard of people putting in some sort of dividers/ splitters, but I am not sure how that is done. I also have heard of putting a "lip" around where the parting line is to be joined with a certain type of screw for better securement later when pouring the wax. I know I'm not very clear (which is why I need you!) It depends on the size, complexity of the model and so many other factors. I never like CUTTING molds in half, they never fit right, you need to MAKE the mold in halves flanged, keyed so the two halves of the rubber and plaster have a wide mating surface with which to seat together. Somewhere I have a photo maybe but it's 2:30 AM and I need to get to bed!
Also, the silicone heats as it cures and I am wondering if it is going to bust/ crack the fired piece. First you'll need to stop thinking in terms of pouring rubber and heat from that huge mass, brushing on rubber is a whole different world- the QM140 does NOT even get warm!
Perhaps I should try this thixotropic silicone you mentioned rather than using my "leftovers"...???
I would unless you have a large amount left over, but even so it doesn't sound especially suited for this particular project, use for another maybe.
I'll check on the "smooth on" how to. Thanks for the advice.Smooth-on was from someone else, good info, but years ago when I tried their RTV rubber I didn't like it, the QUantum which someone here recommended to ME a while back has not given me one problem. The big thing is in brushing on, be SURE you get enough thickness on the corners and edges because it can fool you into thinking you have plenty on those spots and it turns out you dont.
06-07-2006, 12:58 AM
Ok, couple of pics showing a flange for a 2 half mold, here's the mold to start with assembled;
It was a use once mold as all I needed was one plaster cast from it to work on.
Below it's taken mostly apart, foreground has the left side and it's shell still together, in the background the right side is apart showing the 2 smaller inserts around deep undercuts still laying in location on top of the rubber, and the half shell behind it. Out of view is the base it all stands on.
The flange is about 1" wide or so, keyed only a little here and there as I mentioned it was a use once mold. If done well, the two rubber halves will fit nice and snug with a perfect mating on that flange- leak free and seam free. You can't do this really with latex due to the shrinkage- that seam would soon open up a good quarter of an inch.
This was done with NO shims, metal inserts, flags or whatever, the hard model was simply bedded down on a board and plasticene packed around it till one half of the model was covered, the rubber of one exposed side made, the plaster shell of that side made, the whole thing flipped over and the plasticene removed off the one side.
Then the rubber made of the second half when the plaster shell on the first half was dry (just to be sure) then the second half shell made.
06-07-2006, 01:11 AM
Embedding model in plasticene to do the first half of the rubber mold, it should be easy to tell that if the rubber is applied over the entire area a wide flange will result;
06-07-2006, 01:53 PM
Good pictures. What method would you prefer if the piece is itself made of plasticene or wet clay, and can't be imbedded in it without ruining that side? Standard metal shims approach?
06-07-2006, 04:54 PM
Sorry I missed this when you first posted, but my phone been out for several days. Landseer and anatomist have given you a great overview of the preferable approach, and good detail. “Thixotropic” materials are naturally (artificially-induced) gloppy, and they will stick to anything, up to a point, so they are excellent for use in painting onto vertical or even overhanging surfaces, such as the nose or chin of a face.
What I‘m not sure about is painting anything of this sort onto fired clay. You need to be sure the molding material doesn’t get permanently stuck onto your original. Also, anything you use as a separator must be paper-thin so as not to distort the original form. There are spray-on separators, and most I am familiar with are silicone-based, but they typically are for use with water- or oil-based, unfired, clays, so I don’t know how they would work with your fired clay.
I would suggest you talk with Smooth-On or another supplier I have heard about on the grape-vine, Permaflex. They are comparable in quality and pricing, from my secondhand experience, and both carry a full range of molding, and possibly also casting, supplies. Typically companies of this sort are more than happy to help users with technical questions
Please ask any more questions. This thread will help many people. And, I think Landseer missed the point that your foundry seems to be not far from your own location, i. e., is inside the U. S. I guess the US still can be competitive in some processes.
dont know if this is any help to you but you can, paint on the thixotropic silicon as described above then back it up with fibreglass and resin to keep it nice and light.
dividers can be made of virtually anything, clay walls are the norm but as your piece is dry they will not stick most likely. metal shims can be used also but need to be pushed into the piece.
so i think your best option is using a thin line of silicon and sticking metal/plastic shims in it when its not cured yet. you may have to re-work the seams a bit more than usual but i reckon that should work.
alternatively you could use polyurethane rubber as it is alot stiffer (for the edges) but i dont know how it would react being in contact with the silicon.
using the silicon and fibreglass method is also cost effective (as much as it can be) because you use hardly any rubber.
i have seen this method in action in a bronze foundry and the silicon is barely ever thinker than a couple of inches at the very most.
hope that helps
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