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Plaster Mold Making & Some Ceramics
Grand Member (first year)  

Joined: Sep 29, 2003
Posts: 993
From: near Atlanta, Georgia
Posted: 2008-05-03 09:29 am   Permalink

Great thread guys! I have been messaged with a few questions as well so will tack on some info here. i usually use Lagunas water based (em210?) white clay for building a separation wall. When using water based clays for sculpting or molding purposes, there is always the fine line of too wet too dry. One technique that helps in the last final smoothing your piece or parting wall is to use 99% alcohol (or 70% is thats what you can get) instead of water. This will not saturate the clay like water, will evaporate faster, and helps to prevent sludging the surface. A great tool for smoothing water based clays is a foam rubber cosmetic sponge. When bulking out the form and getting to the early steps of smoothing before final detailing, a powder puff with water is a great tool.
Working out "thumbiness" in a sculpture early on is key. A lot of people want to rush to detailing, thinking it will hide uneveness or bumpiness, but it usually does not. Get your bulk shape as clean as you can before adding detail. With oil based clays, use some window screen, the thicker waxier feeling stuff, wrapped on the tip of your sculpting thumb, and work the surface to take out that wavy thumbiness.
If you are just begining as a sculptor, start with water base. If you get a good feel for sculpting in water based clay, you will have a step up when starting to work with oil based clay.

Hope this is clear helpful info, and answered some PM's.

[ This Message was edited by: oceaotica 2008-05-04 10:19 ]

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Tiki Socialite

Joined: Nov 19, 2006
Posts: 2511
From: Lemon Grove
Posted: 2008-05-04 12:21 pm   Permalink

OceaOtica - Really good post! I'll have to remember to try the alcohol thing. I love the fact that other folks are jumping in and sharing their own knowledge. In fact, and I'm sure Bowana would agree, I'm going to change the thread name to "Plaster Mold Making & Ceramics Information" in hopes that other TC'ers will follow your lead. Also, I totally agree with you that it would be really helpful for beginning sculptures to start out playing with regular water based clay first...Hell, for that matter, there is no reason you can't make a mold off of a glaze fired ceramic work too. You might loose a little detail this way, but hey...

If you are just beginning to play with sculpture, you might also consider jumping into a beginning clay class at your local community college. These classes are always fun, and I think one tends to learn quite a bit in them...Plus they are cheap! One of the best teachers I ever had was in a community college setting...For me, I feel I learned just as much there about clay as I did in the grad school I transferred to later on.

Cammo, your right. Your mug shrunk only 6%. I hadn't put a ruler to it before...it just felt double that to me. I put the master sculpt of Oki next to a fired finished Oki here so you can all see what 6% looks like. A 6% reduction in size may be a concern to someone whom needs a mug to hold a specific amount of liquid. Like you said, it's really up to the person. Henrik and Kaimuki, I would bet your are seeing a higher shrinkage rate than this 6% as your final firings are much hotter? A really, really important point to consider when sculpting your master is "how does it feel in your hand"...how will it feel when it shrinks a little?

This picture below is also a good picture, because it shows how the master oil clay sculpt weathered the whole mold making process. Not bad, huh...one could quite easily do another mold from this master without having to do much to repair it. I have seen where other people are using their master sculpt to cast a harder master resin copy first. This way, one could build multiple plaster molds for mass production. There is a good thread showing this somewhere on TC...hopefully someone can post a link to it on this thread? I can't seem to find it.

Another thing, If you look back on the pictures, you will see that we built the finished master up around the top with our oil based clay. This extra space added is needed. When you pour your slip into the molds it shrinks down from the top as the clay is starting to settle and harden in the mold. The clay thins in this "extra space" area. After you have pulled the slip casting from the mold take a knife and trim this extra off the lip of the mug. Dip your finger(s) into some water and smooth out the rim. Be gentle...keep your rim rounded. Remember liquid needs to pour from your mug and look good doing it.

Kaimuki - Love your new mug! You might also want to test a lower fire slip too? What type of problems are you seeing with the cone 5 slip? "Babalu - do you make your own slip" - Nope, I didn't make my own slip for these mugs...I guess the reason is because, it's really easy to just go buy the pre-made slip quite cheaply, and this store brought "pre-made" seems to be always made well. Cammo found some gal here locally (San Diego) that is making slip for a reasonable price too. I think if I were really into to punching out a large number of mugs (100's), then I might make my own slip as well as making multiple molds to use for one design.

Mad Dog - No worries at all. I think it's cool that someone has starting giving this whole mold making thing a shot.

I did a quick online search for a simple tell all about loading a kiln, firing, trouble shooting glazing, etc...I grabbed these two without looking around too much...there is tons of info online these days about ceramics. We are only just touching the surface....


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Grand Member (first year)  

Joined: Jun 21, 2006
Posts: 7209
From: Tujunga
Posted: 2008-05-04 12:55 pm   Permalink

GROG see in the pictures that you poured the slip in the mold and then poured the extra slip out of the mold once the slip hardened enough to give the mold some thickness. About how long should the mold sit for the slip to set up before you pour out the extra slip in order to get a nice wall thickness for the mug without making it too thick or too thin? Now, the slip dries around the outside of the mold inside the impression area because the mold draws the moisture from the slip, thus forming the wall thickness of the mug. Does the room temperature and humidity make a difference in how long it takes for the slip to set up? After the extra slip is poured out, how long should the slip dry inside the mold before you can remove the greenware? How long should you let the mold dry out before you do another pour?

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Tiki Socialite

Joined: Nov 19, 2006
Posts: 2511
From: Lemon Grove
Posted: 2008-05-04 8:38 pm   Permalink

Hey Grog,

Love the humidity question...I can tell you have done this before It takes longer for clay to dry in humid conditions verses dry conditions. Those of you who live in places like Florida, where you can cut the water in the air with a knife, are going to find that you have a harder time drying work than those of you living in say...Denver, where the air is very dry. I grew up learning about clay in Colorado...Things dry super, super fast there. Not sure about the temperature part of your question Grog, but I can imagine that you would not want to try doing a casting where it might be too cold...no freezing temperatures.

Time it takes for the slip to harden in mold:

* I've talked to a few people how have different times for this, but I kind of like my mugs to be just a little thicker than the norm...they just feel juicer to me. I let the slip sit in the molds for about an hour before I pour it out.

Time to let the slip cast dry in the mold:

* Generally, when you see the clay start to pull away from the plaster a little, it's time to open the mold. I have kind of fallen into a routine with my mugs. I fill and pour them out right before I go to bed at night...I let them sit inverted all night...In the morning I open the mold and remove the slip cast.

When is it time to dry molds:

* I usually sit them out in the sun to dry for a day after about 4 or 5 pours. I have heard that some folks like to dry their molds out in an oven at about 200 degrees f. for an hour or so too. I have also heard of folks popping their wet molds on top of a hot kiln to dry them out

Richard Burket is head of the Ceramics Department at SDSU...cool dude. Took one class from him when I first moved to So Cal...here's what he has to say about the topic...

Richard Burkett - SDSU 1993

1. Make sure that new plaster molds have dried sufficiently. It
will take 3-4 days at the least for them to be dry enough to use,
unless you leave them in the sun all day, or dry the molds in a
warm place. Be careful with forced drying, as plaster will start
to deteriorate above 130° F or so. Make sure the mold is clean and
free of all oil, mold release, or grease.

2. If the mold is completely dry (no dampness whatsoever) it
should be slightly dampened before slip casting. Do this by
running it quickly under water (or dip it in a bucket of clean
water) about 5-10 minutes before you want to pour the mold. If the
plaster is not slightly damp, the slip will cast with varying
density (harder on the outside where the dry plaster quickly draws
the water out/softer toward the inside) and cracking will be more
likely. Air bubbles and porous casts are another possibility when
casting in a dry mold. A damp mold will also make dusting more
effective (see step 4).

3. Dust the mold if necessary. Talc is fine for lowfire casting
slip. Silica is better for porcelain casting slip, as the talc
will melt on the surface during firing. Use a pounce made from
thin cloth to lightly dust the interior of the mold. Too much dust
and the fine details in the mold will be lost, and the casting may
come loose prematurely and collapse. This dusting allows the
casting to release from the mold earlier and more easily than it
would otherwise and is not always necessary with simple molds.
NOTE: Wear a dust mask and use the glaze spray booth for proper
ventilation when using Talc or Silica dust to avoid dust

4. Fasten the mold together tightly with large rubber bands,
pieces of rubber innertube, or heavy string tied, twisted, and
wedged snugly about the mold. Be sure the mold is quite securely
fastened, as the slip pressure inside even moderately-sized molds
is surprisingly great. With very large molds, be sure you will be
able to drain them when filled and heavy with slip.

5. Make sure that the casting slip is well blunged (mixed),
free of lumps, and of the proper consistency.

6. Prop the mold upright with a little clay if necessary, and
have a bit of clay ready to patch leaks. Pour the casting slip
into the mold quickly, filling the mold to the very top. As the
slip level drops in the mold, keep refilling the mold with casting
slip. If the mold has a short sprue, fixing a large funnel into
the sprue with a little clay may cut down on number of times you
have to refill the sprue with slip. Do this for 5-15 minutes. The
actual time will depend on the mold, slip thickness, and how thick
of a casting is desired. Check the thickness by pouring a small
amount of slip out of the mold and cutting through the casting
with a needle tool.

7. Drain the excess casting slip from the mold. You may have to
use a knife to open the sprue if it has cast shut. Do not to cut
into the plaster! After most of the slip has drained out of the
mold, prop the mold so that the sprue is down, and all remaining
liquid casting slip continues to drip out. A good method is to
place two sticks across the top of a bucket and prop the mold on
the sticks to that it drains into the bucket. Allow the mold to
drain upside down like this for at least 15-20 minutes. Avoid
shaking the mold! This may loosen the casting from the plaster
and cause it to collapse.

8. Allow the mold to dry until it can be disassembled without
tearing or collapsing the casting. This drying time will depend on
the wetness of the mold and the thickness of the casting. Typical
time from pouring to removal from mold is 1-4 hours, occasionally
it takes overnight.

9. Clean up!

Richard's web Site -

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Grand Member (8 years)  

Joined: Mar 30, 2008
Posts: 9203
From: The Anvil of the Sun
Posted: 2008-05-05 08:44 am   Permalink

What I'm doing will never pass as art but hey, I'm having a good time! I made a new mold with a separate bottom piece to accomodate the undercut rim on the bottom of the mug. When I tried to remove my model from the mold, the mold broke But it broke "cleanly" in half in the same direction as my separation lines so I said "What the heck? Now I've got a 4 piece mold instead of a 3 piece mold". I cast a mug last night and it seems to have come out OK. I'm still using the Cone 5 slip (since I already have a gallon of it) so I won't be able to have Babalu fire it for me but I have other options available. Thanks for the input guys, I'm ready for you to post about glazes.
I just found out that MD 20/20 stands for Mogen David and not for Mad Dog. My TC name AND my teenage years are a sham :(

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Tiki Socialite

Joined: Jul 01, 2007
Posts: 402
From: where the road and the sky collide
Posted: 2008-05-06 10:30 am   Permalink

Great info....mold making is an art in itself eh? Nice battalion of tools you showed us. Thanks for doing this, I've learned a lot. You didn't make it look easy, but easier than what I knew.

The little bit of experience I had making molds ( and I had help) We would pour the mold, and set a timer. then take the piece out and cut it in half to check the thickness. Then do it again until it was what we wanted. I always wondered about the timing and the absorption as the mold got wetter....how to account for less and less absorption if doing multiples in a day.

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Tiki Socialite

Joined: Jul 01, 2007
Posts: 402
From: where the road and the sky collide
Posted: 2008-05-06 10:35 am   Permalink

oops double post

[ This Message was edited by: Robin 2008-05-10 18:56 ]

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Grand Member (8 years)  

Joined: Mar 30, 2008
Posts: 9203
From: The Anvil of the Sun
Posted: 2008-05-10 1:33 pm   Permalink

Bowana had recommended a lowfire Cone 04-06 slip so I took the middle ground and got some Cone 5. I just found out that Cone 5 is not the same as Cone 05, Cone 05 fires at a much lower temp. I've been building an army of mugs that need to be fired on the surface of the sun! No big deal, slip is cheap!

EDIT - Maybe we can have a quick lesson in the differences in types of clay? Advantages and disadvantages between lowfire, midfire, and highfire?

Anything worth doing, is worth doing to the point of wretched excess.

[ This Message was edited by: maddogmike 2008-05-10 20:09 ]

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Tiki Socialite

Joined: Nov 10, 2006
Posts: 1183
From: La Mesa, CA
Posted: 2008-05-15 10:15 pm   Permalink

You'll save a ton on your electric bill using low fire clay instead of high fire! I'm not really sure myself about the other aspects. Sir Babalu?

That's my story and I'm sticking to it.

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Tiki Socialite

Joined: Nov 19, 2006
Posts: 2511
From: Lemon Grove
Posted: 2008-05-15 11:18 pm   Permalink

Sorry Mad Dog, I didn't see your question before. Your cone 5 slip will shrink more than the lower fired slip, but it will also tend to be a stronger mug than the low fire ones will be. There is not a ton that is different, cone 5 clay will be more vertrifyed and will not be able to handle all the different colors that a lower fired clay will. Below is a discription of clay bodies that I pulled from an old Clay 101 class I had on file...kind of long...but hey, I guess I will see you at my studio sometime this weekend.

Clay is a compound of minerals and organic material resulting from the natural decomposition of certain igneous rocks (for example, feldspar and granite are two common sources of clay minerals). As these mineral deposits age and get moved around by natural forces (wind, water, glaciers), chemical changes occur which cause the materials to become clay. The two major classes of naturally occurring clay deposits are primary clays and secondary clays. Primary clays are those that remain at the physical location where the parent rock decomposed. These clays tend to be the most pure, but tend to be less plastic than secondary clays. Secondary clays are deposits that have been transported by wind, water, or glacial activity. These clays, while still very pure, generally have had other materials introduced into their basic composition that change their performance characteristics (color, plasticity, etc.)

Types of Clay

Clays can be grouped or classified several ways: according to the way they are found in nature, by their physical and chemical properties, by the way they are used tomake finished properties, and so on. One of the first ways to classify clay is by the methods used by Mother Nature to create clay deposits. The major types of naturally occurring clay are as follows:

Kaolin or China Clay

Chemically known as Al2O3-2SiO2-2H2O this clay is almost pure white as a primary clay and slightly less white but more plastic as a secondary clay. These clays are a major component of most high-fire porcelain clay bodies and are frequently used in stoneware to lighten the fired color.

Ball Clay

These are secondary clays that have been transported to swampy areas where organic acids have broken down the mineral particles to ultra fine size. These clays are extremely plastic – if used alone they will shrink quite a bit, causing severe cracking.

Earthenware Clay

Earthenware is the most common surface clay found throughout the world. These clays usually contain high amounts of iron, which gives the fired wares the characteristic terra cotta color. True earthenware clay cannot vitrify, which means the clay body remains porous after firing.

Stoneware Clay

Stoneware tends to be kaolins that contain more impurities - usually calcium, feldspar, and iron - resulting in clays that have finer particle sizes and higher flux content. The flux materials cause the clay to vitrify at lower firing temperatures.

Fire Clay

Similar to stoneware clays, fire clays generally contain less flux (especially calcium and feldspar). Fired alone these clays won’t fully vitrify – even at high fire temperatures.

Bentonite Clay

Bentonite is formed from the decomposition of volcanic ash. Bentonite has the finest particle size of any natural clay. It is very useful as a plasticizer but it must be used in moderation – too much of it in a clay body will result in cracking during the drying process.

Slip Clay

These are naturally occurring clays that have a high iron content. At high temperatures these clays melt to form a glaze; no additives needed. Pure natural clays almost always have shortcomings – in a potter’s eyes. The production processes and ultimate use of the piece dictates the properties the clay needs to have – either during the forming stage or as a finished piece. Aclay body is a mixture of clay and other materials designed to meet the needs of the user. Design objectives for a clay body may involve making it more plastic during the throwing stage, improving the body’s stability in large-scale work, helping the body resist thermal shock from firing, and improving general properties such as vitrification and density. The “other materials” in a clay body perform a specific function to make the final product work better for the clay user. Ageneral description of each component and its purpose are discussed below.

Components of a Clay Body

All clay bodies involve combining clay (many recipes call for several types of clay) with non-clay additives. The basic types of additives and the purpose for each are as follows:


These materials act as melting agents, helping to lower the maturing temperature and assist in the formation of glass - the essential binder in all ceramics. Some clay contains higher concentrations of fluxes naturally–feldspar and iron are the most common.


These materials react with fluxes to form glass. The most common glass-former is silica. Pure silica melts at very high temperatures – the proper mix of flux materials and silica allows for glass to be formed at more manageable temperatures. This balance must be carefully achieved – too much flux produces a weak glass, too much silica can lead to reduced thermal shock resistance.


These materials stabilize the body, providing the physical

Hope you can make since of all this...


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Grand Member (8 years)  

Joined: Mar 30, 2008
Posts: 9203
From: The Anvil of the Sun
Posted: 2008-05-16 07:40 am   Permalink

Thanks Babalu, very informative. As I understand, the higher fire clay is more like glass - strong but brittle, and glazes are limited in higher fired clays.

I will give you a call sometime this week.
I just found out that MD 20/20 stands for Mogen David and not for Mad Dog. My TC name AND my teenage years are a sham :(

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Tiki Socialite

Joined: Nov 19, 2006
Posts: 2511
From: Lemon Grove
Posted: 2008-05-16 3:39 pm   Permalink

Hey Maddog,

Sorry man, no, there are a kazillion + 1 glaze choices for high firing clay, and many out there feel that high firing clay is the only way to fire. Low firing "up to" cone 6 offers up some very bright coloring that is harder to achieve in higher fire scenario (over cone 5-6), but that does not make low fire any better... Gas firing kilns and reduction can act differently than firing with electric kilns..I will try to explain more when we hook up this weekend.

Call first before coming over, Amy's bro is in town this weekend and there is a chance that I might have stepped out for a minute.


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little lost tiki
Tiki Socialite

Joined: Jun 12, 2006
Posts: 7750
From: Orange,CA-right near the Circle!
Posted: 2008-05-17 1:59 pm   Permalink

Great thread!
You're like the CLAY WIZARD!
I can't wait for the future so
i can download ALL of the info provided
into meh Brain!
Thanks so much for sharing so much!

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Tiki Socialite

Joined: May 18, 2006
Posts: 2028
From: San Diego
Posted: 2008-05-17 2:54 pm   Permalink

When we made the Oki Doki mug, I basically just wrote all the drying and firing info that Babalu told me on the side of the plaster mold with a sharpie pen. It's still there, scratched and smudged.

The best way to learn all this stuff is to just full speed ahead do it yourself. Mistakes are fun! How do ya think Muntiki did those zombie skull mugs?

I was thinking - what about a mug making workshop at Oasis this year? We could do every stage from sculpt to mold to pour to fire, once a day, three days in a row. We could take pictures and post daily...

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Tiki Socialite

Joined: Nov 19, 2006
Posts: 2511
From: Lemon Grove
Posted: 2008-05-17 9:48 pm   Permalink

I got to meet Mad Dog Mike this morning...nice mugs mike! Look forward to what you come up with this week.

LLT - Thanks for jumping in...you need to come down and make a big clay canvas to paint on. I need to make the trip up to your place too...I have to see your little center of the universe first hand.

Cammo - That Oasis demo thing is quite possibly the best worst idea I have ever heard...did you take your meds this morning brother?
Babalu can't sculpt, make mold, dry and fire a mug per day....Glaze those Okis you have dude...soon. Hope you guys had fun at the Chop.

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