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Art-Deco 11-03-2014 12:51 AM

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 80º/hour to 200º and candle for 9 hours.
60º/hour to 1200º which moves it through the water burn-out and quartz inversion stages nice and slow.
After that it goes 80º/hour to 1950º and ends with a 10 minute hold at 1950º @ 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 @ 2,056º so that will take 1-1/4 hours longer for the firing.
I expect the sculpture will weigh about 38#

Art-Deco 11-09-2014 10:16 PM

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.

raspero 11-10-2014 06:40 PM

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.


Art-Deco 11-10-2014 10:05 PM

Re: New process

Originally Posted by raspero (Post 106216)
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.


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.

Art-Deco 11-11-2014 01:16 AM

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.

rika 11-12-2014 06:22 PM

Re: New process
Nice to see your process, and glad it worked out so well.

Art-Deco 11-18-2014 10:45 PM

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.

mantrid 11-21-2014 11:38 AM

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?

Art-Deco 11-22-2014 01:10 AM

Re: New process

Originally Posted by mantrid (Post 106239)
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?

That photo is a Densite plaster cast Mantrid.
Yeah I have some pics, and for sure, there's lots of places to trap air in this!

I use both a water reducer and I use the same technique I use with plaster- put on a rubber glove and dive into the face of the mold and work my fingertips into the details to dislodge the trapped air, it works perfectly with plasters, with concrete it's more difficult as the stuff is dense and settles in which makes it harder to even get your hand down in let alone move it around much.

The first picture is the unfired clay original, unfortunately I had cut the head off and hollowed it out in 2007 when I made it, and I didn't do a great job melding the head back onto the neck and at some point the head came off during handling it to my car to take it over to my gallery building a year ago, the head fell off and dinged the lower law against the car door frame and damaged it, so I had to fix that damage and the seam with some plasticene and spackle.

Pulling the rubber mold off the glued-on head predictably came off and the ears broke off, but the body and paws were not injured at all.
The original model I guess is pretty much "toast," I suppose I could re-attach the head and ears with that patch-attach stuff for greenware repairs, but the seam in the neck and the damaged lower jaw are pretty much unfixable to do it to fire the piece.

I do have the plaster cast however, the cast by the way wound up being 90# when dry, I had a fan on it up till now, so 10# of water evaporated out and it feels dry now, so now I know these will average about 90# for the plaster version.

I'm leaving one section of the mold shell out initially- the section to the right of that shim line in the last photo above, that will let me lift that "flap" open and access the head and face through the shim opening, and then once I've worked the air out of the face, ears and head and fill the head up, I can slip that shell section in and strap the mold tight and continue filling it.

The plaster cast is now stained.

The toes on the original number and odd three per paw as my model does too, why three toes? I have a theory that the original artist decided to accentuate the strength of the claws and toes to give it strength and power, that's supported in part too by the muscles in the forelegs, the massive power of the paws are the first thing that draws the eye on this, to fit four toes on each paw they would have had to have been modelled smaller, and closer together, the impact of that would have been significantly reduced.
I searched for what the design might be called in mythology, closest I could find to this is a winged dog in Armenian mythology called an Aralez. It's almost certain with the waves of immigrants from Europe, Ukraine, Armenia etc in the 19th century that they would have worked in a terracotta factory such as the one that made these and influenced the designs.
It's not a griffin or a winged lion, a winged lioness would be doubtfull.

Art-Deco 11-23-2014 01:18 PM

Re: New process
I placed the first pressed clay in the kiln about 45 minutes ago, I adjusted the kiln schedule a little from the previous tests due to wanting to reach a higher temperature than 1950º

R1: 80º/H to 200º
Hold for 9 hours @ 200º
R2: 60º/H to 1200º
R3: 90º/H to 1700º
R4: 80º/H to 1950º
R5: 60º/H to 2079º

The 9 hour hold @ 200º is the real important one, why 9 hours? it was just an arbitrary number, it worked before, it could just as easily have been 10 hours or maybe less, but I felt it better to leave it in longer than needed than not long enough and blow up.
A similar sized panel was fired a month ago, but it was completely solid, it survived fine but it had been sitting around since 2006 so it was as dry as it ever would be all the way though.
This panel was made 12 days ago, feels dry and has been in front of a fan for the last 2 days too, it's hollow and slightly larger, but the same 9 hours should do it.
The slow 60º/H rise to 1200º is probably more time than needed but it gets it going slow through the water burn-out and inversion phases, once it's reached 1200º it's out of the woods and I set it to a moderate 90º rise then slowed it down to let the heat soak all the way in through the whole mass to final temperature.

It should take about 38-1/2 hours.

My Olympic kiln, purchased new, is not the large oval I wanted, but it's a reasonable size, pieces like this panel have to set in it on end, a half shelf resting on tall posts not seen in the photo is set in place between elements and touching the wall, the panel just touches the shelf edge with one corner to give it some vertical stability in case it wants to tip.
I had one smaller, narrower panel set on edge early on which did start to lean and it tipped over enough to touch the brick wall, unfortunately it tilted enough of an angle that it warped a little. I didn't use the shelf idea on that at the time.

What I need is that larger Olympic oval kiln, but it was about $1000 more as I remember, maybe not quite that much, but quite a bit more. I would also have to replace my electrical panel if I bought it since all the breaker slots are full, it does not accept half height breakers, and the main breaker is only 100 amp, the oval kiln wants 70 amps if I remember right.

Art-Deco 11-25-2014 01:42 PM

Re: New process
I took the panel out of the kiln now, it came out very good. I made one small error in the final temperature 2079º I should have set it to 2060º, that 19º difference seems minute in a kiln over 2000º but it makes the difference between the clay turning slightly brown and staying the red color I am aiming for. I had forgotten the higher temperature gave me the browner color a year ago when I fired another piece to that temperature as a test, and then lowered it to 2060º and the red color remained.

The photo doesn't show the color all that accurately, so I placed another piece fired at 2060º previously in front of it for comparison. It still doesn't show it well but it can be seen there is a difference.
The lion is a redder/orange while the panel is in real life slightly red/brown. This clay will turn a darker chocolate brown at a higher temperature but I that's not the color I want. So on Thursday morning I'm going to fire the second panel since it's dry now enough to fire, and fire it at 2060º

Art-Deco 11-30-2014 02:52 AM

Re: New process
I finished the mold for my latest work and here's the first cast in tinted plaster, 23x19"

And I have the concrete cast done for another client, here it is fresh out of the mold, photo taken before wrapping in plastic for a few days, it gets acid stained and then ships to Nashville.
It weighs 150#

raspero 11-30-2014 06:13 PM

Re: New process
This is fascinating. The molding is not so far from what I do, but yet very different—working in fired clay and concrete. It's a different world.


Art-Deco 12-01-2014 01:16 AM

Re: New process

Originally Posted by raspero (Post 106257)
This is fascinating. The molding is not so far from what I do, but yet very different—working in fired clay and concrete. It's a different world.


Some seem to like fiberglass for mother molds, I don't, I normally just use whatever casting plaster I use for casts, used to be Hydrocal but the past few years I use a rival brand to USG called Densite, and it's stronger and harder. But lately when I get a shipment of Densite I order 2-4 bags of pottery plaster to use for shells, it's goes a little further per pound and it's slightly lighter.

I also took the second fired panel out of the kiln, they are coming out good, no cracks or warp but I'm finding two issues, one is, setting them directly on the floor of the kiln is not allowing that area of the sculpture on the edge, going in about an inch or so develope the same color as the rest, it's slightly lighter. So to correct that I'm going to place them on some posts laid flat so they are about 1-1/2" off the floor of the kiln.

The other issue was even reducing the temperature from 2079º to 2060º I'm tending to get the brown color more instead of the red, a ∆1 I put in on th emiddle shelf actually bent over, so it would seem the kiln is firing hotter than what it's set at, so i've made a small adjustment to the firing schedule, reducing the temperature 10 degrees to 2050º and changing the 10 minute hold to 5 minutes:

R1: 90º/H to 200º
Hold for 9 hours @ 200º
R2: 60º/H to 1200º
R3: 100º/H to 1860º
R4: 60º/H to 2050º
Hold for 5 minutes

Next firing I do I'll see how that change works out, I guess I will get some ∆1 cones and some ∆01 cones and start monitoring to see. The 01 should bend way over and 1 should not bend, if it does it's getting hotter than I want.

Blake 12-01-2014 08:50 AM

Re: New process
Great post!
Unlike Richard, this is very different from what I do, but fascinating to see a professional at work.
Your photos of the molds are wonderful, so interesting to see the process.
Thanks for sharing

Kind Regards

Art-Deco 12-02-2014 09:34 PM

Re: New process

Originally Posted by Blake (Post 106262)
Great post!
Unlike Richard, this is very different from what I do, but fascinating to see a professional at work.
Your photos of the molds are wonderful, so interesting to see the process.
Thanks for sharing

Kind Regards

Thank you Blake!

I ordered a Bartlet digital 120vac pyrometer and a box of ∆1 and ∆01 cones today, so next firing I will know for sure exactly what temperature I'm getting.

Art-Deco 12-14-2014 12:27 PM

Re: New process
I have the new pyrometer installed and I fired the 3rd panel using the slightly modified kiln schedule I mentioned I would try- lowering the temperature 10º and setting the model in the kiln on some posts laid flat on the kiln floor so the model was not resting directly on the kiln floor this time (had affected the color a little on the edge that was touching the floor as it didn't get as hot as the rest)

Looks like both issues have been fixed by the change. The left panel was the previous fire, the temperature just reached the point where the clay starts turning more brown than red, and the lighter reddish "band" on it's right was the side resting on the kiln floor which didn't reach the higher temperature so it stayed redder.
The panel on the right was the latest one, and it has the red I wanted to get, and the whole fired to the same temperature since it was resting on 3 posts instead of directly on the kiln floor.

The terracotta sure beats concrete in every way- appearance, quality, durability and lower weight. I did a boil test on a small test piece using this clay and from what I was able to determine, the absorption of this clay was around 3% or less which is even better than the standard for hard building brick, no doubt due in part to firmly hand-pressing the clay and compacting it well.

I want to buy a larger, oval kiln but unfortunately it looks like to run the model I want with the 60 amp breakers it needs, I'd have to replace my main breaker box up to and including the meter socket box since the meter to panel cable is not sufficient in ampacity to install a 150 or 200 amp main breaker panel to replace the current 100 amp. The current box is also full and can't accept any more breakers either.

The Olympic oval line has a 25"x37" model that would work fine to lay stuff like this panel inside, flat.

I have another design I want to do this same process with, but with the shrinkage of the clay and the smaller starting size of that panel, I won't be happy with the reduction in size on it, so it means I'll have to enlarge the original design to compensate for the shrinkage of both the clay model and the clay pressing, but also make it larger yet so the end product will wind up larger than the current version.

I will have to calculate out what size to print out the reference sheet at to use to sketch out the design on the clay.

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