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  #1  
Old 11-02-2014, 11:51 PM
Art-Deco Art-Deco is offline
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New process

I decided to replace the original lousy Quantum rubber mold made in 2007 of my design, with a new Smooth-on mold since the Quantum rubber since the stuff was so poor it tore in all four corners within the first couple of casts made from it. I've been delicate with it and babied it along as well as repaired it numerous times.
So the photo is my plaster master set up to make the new mold:



The nice, strong Smooth-on mold all finished and a release applied to it for the next step:



The next step wasn't photographed as there wasn't much to see, but I made a Smooth-on positive mold of the above negative mold with 3-4 brushed-on applications to get it about 1/4" thick. Once I applied the last coat and it started to firm up, I filled the cavity with plaster and let it set:



Once it set I turned the whole affair over and carefully removed the negative mold rubber with it's four piece plaster shell from the rubber positive with it's plaster in-fill. Once removed I was left with the rubber positive with it's plaster mother mold in-fill supporting it inside:



Some might already know where I'm going with this next, for those who don't- the next step is making a form around it and casting what will need to be a five piece mold made from molding-plaster. While there's few undercuts in this design there are some, and future projects like this will have considerable undercuts. The rubbers' flexibility completely eliminates any possible adhesion as well as any issues with undercuts causing problems removing the five piece plaster mold from it.
I could have filled the whole negative mold up with liquid rubber but it would have taken about 2-1/4 gallons of it to fill, thus making it just a skin mold with cheaper plaster in-fill.
The plaster mold when dry will be used to hand-press the course red clay I use into it.
I've already put in my order for some other materials and added 200# of molding plaster so I'll have some on hand, that's supposed to come Friday, so next weekend I plan to make the plaster mold.

I already came up with a workable kiln schedule and tested it on two pieces similar in size to this, one of them was 2" thick solid clay, no cracks or blowouts.
I decided to go with:

Ramp up 80/hour to 200 and candle for 9 hours.
60/hour to 1200 which moves it through the water burn-out and quartz inversion stages nice and slow.
After that it goes 80/hour to 1950 and ends with a 10 minute hold at 1950 @ 36-1/2 hours.

But that was to cone 04, I want to go to what would be cone zero if there was one- as that got the best color @ 2,056 so that will take 1-1/4 hours longer for the firing.
I expect the sculpture will weigh about 38#
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  #2  
Old 11-09-2014, 09:16 PM
Art-Deco Art-Deco is offline
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Re: New process

Continuing with the process, the rubber positive is now set up in a box to pour the five piece plaster mold, the first four sections shown:





And the largest section, which forms the "base" of the mold and has the entire face design on it, it took 4-1/2 gallons of plaster to fill this section:



The mold removed from the wood form and rubber positive, edges bevelled, and then set aside in front of two box fans on high to dry after this photo was taken:



The reddish color is partly from adding a small amount of red tint to the plaster used to cast two sections so they are easier to see where they meet when removing them the first time, and some is from the red clay used for temporary blocking.
I use ordinary, cheap liquid hand soap for the mold release on the plaster, with 3 applications the plaster sections needed absolutely no prying to separate them- they all but came apart by themselves. Why use expensive "mold soap" when the $1 bottle of Landers or Good Sense hand soap works just as well

Hoping it will dry enough by tomorrow night to try pressing a clay cast from it.

The incized lines on the original design means they are thin raised lines on the plaster mold, I can see I'm going to have to be real gentle in order to avoid breaking those off.
I don't expect I'm going to sell "hundreds" of these but the less re-touching up on each clay that has to be done to put back missing details from broken off raised "lines" in the plaster mold the better.
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  #3  
Old 11-10-2014, 05:40 PM
raspero raspero is offline
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Re: New process

I find this an interesting process. Keep posting the coming steps.

What Smooth-On rubber did you use?

I use Smooth-On rubber as there is a distributor up in Mexico City who stocks a lot of the products. I have been using Reo Flex 30 polyurethane. I add Cab-O-Sil to it as I brush it on.

I have used a small amount of silicone, but I have no vacuum pump.

Richard
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Old 11-10-2014, 09:05 PM
Art-Deco Art-Deco is offline
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Re: New process

Quote:
Originally Posted by raspero View Post
I find this an interesting process. Keep posting the coming steps.

What Smooth-On rubber did you use?

I use Smooth-On rubber as there is a distributor up in Mexico City who stocks a lot of the products. I have been using Reo Flex 30 polyurethane. I add Cab-O-Sil to it as I brush it on.

I have used a small amount of silicone, but I have no vacuum pump.

Richard
I have been using the Rebound 25, for about 6 years now for several reasons, one is it's a simple 1:1 mix no scale needed, it's platinum cure and is completely unaffected by water or moisture, in fact i've used it to make a mold of a moist clay model, and that plaster backer poured into the cavity of the positive rubber a few photos up was poured in while the rubber was still tacky.
Smooth-on has said that it's been used under water to make molds.
It's extremely strong and I have had zero problems with it.

I've looked at other formulas of Smooth-on but none of them seem to be right for my use, either you need a gram scale or some property about them I found Rebound 25 better with.
I like the idea of their "sort of clear" rubber but it requires a gram scale and I'm done with the scale stuff after the time I was in the middle of making a large mold and the electronic gram scale suddenly started displaying crazy weight numbers that didn't make sense and I knew was way off.


That Quantum purple colored platinun cure silicone mold rubber I was using about 6-7 years ago turned out to be total garbage in every way.

I use the thixotropic additive to make the Rebound very brushable from the start, only takes a teaspoon of it for a whole bucket of the Rebound.
I do not have a vacuum pump, it would be usefull but running the rubber thru that process shortens your working time since you have to wait for it instead of already applying it to the model.

Now that the plaster mold is mostly dry after two days in front of two box fans on "high" I decided to try pressing the first clay in it.
It took about a half hour to carefully press the clay in, and it took about 35# of clay.
I added the interior webbing that all of the architectural terracotta originals my work is based on- have for structural stength and integrity. My pieces don't need the strength so much since they are not load bearing elements meant to support three stories of brickwork above them, so the webbing and wall thickness is between 5/8" and 1" thick, while the antique originals were typically specified to be 1-1/4" to 1-1/2" thick.

Here's a view of the back, I think in about an hour or two it should be firm enough from the plaster mold absorbing moisture from the clay that it will be firm enough to at least remove the sides of the mold. To remove the clay pressing it will have to stiffen further enough to support it's own weight.
I might just lay a piece of plywood over the back and turn the whole thing upside down on that and then remove the mold rather than trying to lift the clay up and out and turn it over.



I suspect the clay will need a fair amount of touching up, something I know they had to do even in the era when this stuff was made, it might need a little more than it otherwise would due to the fact I was trying not to pound the clay in hard due to the fragility of parts of the design.
In retrospect since this mold is not being used to cast slip in, it's porosity is not nearly as critical- the moist clay is already partially firmed up and only needs a small amount of moisture absorbed out to do the job (unlike slip) I could probably mix in 50/50 densite plaster with the pottery plaster to make it harder and less easy to damage fragile details.

This is the first large piece I've pressed in a plaster mold like this, the three small pieces I tried last summer did work fine, now I'll have additional weight of both the mold and the clay, and more shrinkage to contend with. The little lion blocks and their molds were real easy to work with, turn over, whatever, but this panel is 35# of clay in a mold that's probably about 75# so obviously it's going to need a little different technique and handling.

Last edited by Art-Deco : 11-10-2014 at 10:17 PM.
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  #5  
Old 11-11-2014, 12:16 AM
Art-Deco Art-Deco is offline
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Re: New process

I'm pleased to see that the first pressed clay came out better than expected, only a little minor defects need touching up, here's a photo of it fresh out of the plaster mold, because it's late I've wrapped it in plastic to mess with tomorrow night.

I wound up simply standing the mold on edge, taking the band off, carefully removing the two sides and top and then removing the large base section leaving the clay model resting on it's bottom side on the bottom plaster section. Then I placed a plywood sheet with 3 strips of plywood against the clay models' back and just tilted the whole thing horizontally to leave the clay model supported on the 3 strips on the plywood. The strips act as spacers to allow air to circulate inside as it dries.




I'm guessing a half hour or so of "cleanup" and re-detailing touchup will be what it takes, the small defects are mostly where one bit of pressed clay merged against another leaving a very tiny "line" or wrinkle. Some of that might be controllable by technique, or it might just be the nature of the beast with pressing clay- as you work some of the clay starts to dry and the edge of that leaves a little line when more damp clay is pressed next to it, something like that seems likely.
The main defects can be seen on the upper left part of the smooth "V" and on the bottom edge where the face met the side, that 90 degree edge has some "lines" or wrinkles where the very surface of the clay didn't quite merge 100%

It took about a half hour to press the clay.
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Old 11-12-2014, 05:22 PM
rika rika is offline
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Re: New process

Nice to see your process, and glad it worked out so well.
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  #7  
Old 11-18-2014, 09:45 PM
Art-Deco Art-Deco is offline
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Re: New process

Thanks Rika,
The two pressings are mostly air dry now, hope to fire them maybe in a couple of weeks or so.
They are 20-1/8" long which is quite a reduction from shrinkage from the 21-1/2" plaster version. They may wind up 19-1/2 after firing, so it's about 2" loss.

I finished the mold for my seated winged gargoyle, it took 3 gallons of Rebound 25 rubber and 125# or so of pottery plaster for the shell. I cast the first plaster cast from it tonight, it took almost two 5 gallon pails to fill, and exactly 100# of the Densite plaster.
After hollowing it out as much as I could while it was setting, the wet out of the mold weight is 100#
So if I figure 19 quarts of water used is about 40#, less what was in the waste from hollowing him out, I've noticed about half the weight of water used in plaster casts evaporates, the other half stays chemically bound, so the cast pictured should wind up around 85# which means even crated it can ship FedEx ground.

He is 27" tall and is based on a 1906 design.



A client wants one in concrete for his building, that one will have to go by truck.
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  #8  
Old 11-21-2014, 10:38 AM
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mantrid mantrid is offline
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Re: New process

is this the plaster cast or the clay original? do you have pics of your mould? i can see areas where there would be trapped air pockets. how did you overcome this?
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