Oil Clay Sculpting for Slip Casting Molds

[Hyn Patty]

Not sure if I put this in the right place so feel free to move it where ever staff want it to be!  It's one of the many things I'm working on in my studio at the moment.  As many of you already know, I'm an equine sculptor and I also do custom finish work on other equine artists' sculptures - mostly ceramics.  I have developed my very own style and techniques unique to my work.  I am creeping up on the 50th anniversary of my art studio coming up in 2026 so it is an exciting time for me.

Anyway,  I thought if any of you might like to try your hand at working with oil clay, a simple base relief sculpt is a great place to start.  You can make one or two piece plaster molds for slip casting it.

You don't have to be a great sculptor to get started.  Find a good free use photo of a subject you like or take your own shot.  Be it a dog, a still life, your grandchild, whatever!  Use tracing paper if you  have to and make a sketch of it rendered down to basic lines if you don't feel confident enough to sketch it freehand.  Tape it behind a piece of glass from a small photo frame or get a piece of glass cut at your hardware store.  Tape around all of the edges so there is nothing sharp.  Then start laying in oil clay over it.  Get you an even thickness to begin with.  If you are confident enough to skip the sketch under the glass, use an already glazed ceramic tile. 

NO detail at this early stage.

 

 

Edited January 19 by Hyn Patty
Because I am a dorkface.

[Hyn Patty]

Once built out to a more or less rough shape and even thickness, I cleaned it up a bit to get the general shape over my quick sharpie sketch I actually did right on the glass.  A little alcohol allows me to smooth and also lift off any excess marker.  Or you might have a pencil sketch under the glass.  Just use it as a guideline for the outline of your shape.  Keep it SIMPLE to start.

Edited January 19 by Hyn Patty

[Hyn Patty]

I then layer on thicker portions to push it more and more three dimensional.  If you haven't really sculpted anything before, keep it simple so you don't get frustrated or over whelmed.  You can slowly progress more lifelike and more detailed as your skills progress.  This is great for medallions to mount on a wall as basic relief sculpture. Or you mold and cast it in ceramic to add to pottery.  I've made custom light switch plates this way, for example.  This particular one is going to be a show award for an upcoming event this spring, and another later this summer at the Kentucky Horse Park.  I plan to produce it in porcelain bisque as well as glazed. 

Right now it still needs plenty of work.  I want to soften and refine the mane and then do a lot of smoothing before I will go onto pouring a rubber master or molding.

 

Edited January 19 by Hyn Patty
Because I am TYPO QUEEN!

[oldlady]

beautiful,

sorry hyn, some people without a sense of humor have to be humored by editing my comment.    

Edited January 21 by oldlady
offended someone

[Denice]

I have been watching the progress on your medallion,  can't wait  to see your finished medallion.   I worked with oil clay when I belonged to a sculpture group.   I made plaster molds off my pieces.   My son was into medieval times so I made him a chess set that was as close to medieval characters as my research revealed.   I use Cone 5 black and white slip to pour my molds with.  The only problem I had was that they broke easily,  I don't know if I poured them too thin or it wasn't a good clay for a seven year old boys chess set.  I still have the set and the molds,  I ought to dig them out and make a new set.   I have a block of  oil clay in my studio,  I split a order with the man who was the featured at Silver Dollar City  sculpture cabin every summer.  I enjoyed working with the oil clay,  my only problem was that the clay would get soft from my hands holding  the chess pieces while I worked.   When they got sticky I would put them in the refrigerator  and work on a different cooled  piece.   I am happy you are back to sculpting,  I should probably look around for victim and sculpt a bust of them.       Denice

[Hyn Patty]

20 hours ago, Denice said:

I have been watching the progress on your medallion,  can't wait  to see your finished medallion.   I worked with oil clay when I belonged to a sculpture group.   I made plaster molds off my pieces.   My son was into medieval times so I made him a chess set that was as close to medieval characters as my research revealed.   I use Cone 5 black and white slip to pour my molds with.  The only problem I had was that they broke easily,  I don't know if I poured them too thin or it wasn't a good clay for a seven year old boys chess set.  I still have the set and the molds,  I ought to dig them out and make a new set.   I have a block of  oil clay in my studio,  I split a order with the man who was the featured at Silver Dollar City  sculpture cabin every summer.  I enjoyed working with the oil clay,  my only problem was that the clay would get soft from my hands holding  the chess pieces while I worked.   When they got sticky I would put them in the refrigerator  and work on a different cooled  piece.   I am happy you are back to sculpting,  I should probably look around for victim and sculpt a bust of them.       Denice

*grins at Old Lady* 

Yay, Denice!  Yes, recast that chess set!  Probably just because young boys are hard on things.  If you are up to casting them in stoneware or porcelain that will be a lot stronger than earthenware, though I have no idea what clay you used.   You could even cast master pieces, clean them really well and send them on to be molded and cast in resin or give it a try yourself.  Smooth-on and similar companies have great brush on rubbers you can use, back it with a plaster mother shell to add support, then cast using resins that don't require vacuum degassing.  Fun, fun!

Thank you also for your kind words about my silly medallion.  I have a whole series of them planned but as always, I have about 1,200 projects going on at once...

I do the same with the oil clay I use which is Van Aken's plastalina.  I toss it into the freezer in between sculpting sessions to harden back up, or heat it in a crock pot when I'm claying up initially but as you say it's relatively soft.   There are various harder/firmer oil clays out there you might like to try.  Some of them contain sulphur but that's really only a concern if you need to heat them (they can smell) or if you are making silicone rubber molds.   You might want to order a block of something like Chavant 'hard' which is sulphur free plasteline that is very firm.  So firm that it makes my hands ache when I try to work it but a hair dryer or heat gun can help.  My art studio is in my basement (mountain cabin) though so it tends to stay in the 50's in winter and 60's in summer, which makes it very challenging when I need to pour temperature sensitive rubber or resin...

Thankfully plaster is so much easier.

Edited January 20 by Hyn Patty

[Denice]

I think resin work can be very  intense and deep with color but I will have to pass on it.   I have MS and my immune system thinks it is protecting me when I am around unusual smells,  it actually shuts down my lungs.     It can be anything from a strong cleaner,  bug spray,  spray paint,  mold, cat dander ect.   I carry a EPI pen with me but I usually just try to avoid situations that causes me problems.   I will have to check my bag of clay  and  see what brand I have,  it was my sculpture partner  favorite clay.   He was from a nearby tribe and mostly did southwestern sculptures  a lot of eagles and  cowboy and horses.  The slip I used was called a C5 Porcelain slip,  my supplier had it in black and white,  it was new on the market.   I'll have to look around and see what is new at my ceramics store these days.   Denice

[Hyn Patty]

I've been going back to using porcelain (and also playing around with making my own English fine bone china slip - still testing that!) and one of my favorite porcelain slips was Seeley's.  They went out of business long ago but I still have a jug left.  BUT then I found that New York Dynamic ceramics, where I get some of my china paints (the rest tend to come from Maryland China and Dallas China) sells the original Seeley's porcelain slip as well as the Seeley's china paints, using the same formulas.   Happy squee!

Anyway, I digress there again but yay for porcelain slip.  I need to get back into practice making settles for sculpture that'll slump in firing.  Been a few years and all my old notes are gone, so old dog, refreshing old tricks.  Share some photos of your chess set when you recast it, will you?

Random rambling when I can't sleep. 

[Hyn Patty]

Small update - here is the finished oil clay with a very simple Lego mold box erected around it.  I tend to use Legos (or generic off brand) for tiny things and I made larger mold boxes out of wood or thick acrylic plexi as needed for larger projects.   I have sealed this down with oil wax and then added a bead of oil clay all the way around it on the outside.  I'm about to add a very thin bead of oil clay around the inner edge of the box as well so I can be REALLY sure there will be no leakage.  I will then coat the inside of the mold box and glass backing with oil soap.  I can fill it with water to the depth I want to check for leaks, then pour the water off and measure it to see how much plaster or rubber I will need to make my pour.  

At this point you can see I haven't bothered to lay in a pour hole, sprues for air venting, or even mold keys - but I can!   I'd just want a larger mold box to give me plenty of space to do that, which I have not set up  here.  This pour is set up for a uber simple one sided open mold.  It is otherwise about ready to pour plaster or rubber.   If I did this in plaster I'd want to make a rubber mold of my plaster mold before I started production so I can pour however additional plaster molds I might want or need in the future. 

Another option is I can pour rubber now and skip the plaster.  I can still use an open backed one sided rubber mold to cast a single flat resin.  I pull that out, clean it up and fine tune anything I like, even adding more sculpting in epoxy if needed.   I can get away with undercuts and very thin areas if it's going to be rubber molded for resin production.   I can also cast  a rubber positive master from that rubber mold, from which I can also go on to make plaster production molds.  So there are LOTS options on how to proceed! 

In my case I want to do BOTH so I have to decide which one I'm going to do first, then go from there. 

 

  

 

 

Edited February 22 by Hyn Patty

[Hyn Patty]

Now lastly for a bit of math.  Yes, MATH.  But this is real simple stuff so no groaning!  If I have six small molds I have clayed up and boxed, I need to know how much plaster to mix up to pour them all start to finish with all of their various parts.  Or rubber!  Rubber is especially expensive, running about $100 to $250 PER GALLON depending on the kind of rubber I'm using.  In this case I'm going with Smooth-On.com's Mold Max 40 (the number is the shore hardness).  These are for my masters, including all mold parts and the positives for making plaster production molds from as well.  So by the time I'm done there will be no hollow parts.  Most people can deduct a little bit based on your rough measurements of hollow cavities you won't be filling.

So, how much do I need and how do I figure it out?  Now I have set up all my mold boxes and clayed up for my first pours I have a pretty good idea how large each finished mold will turn out to be.  So they are roughly the following in inches:  4x4x6, 3x4x4, 3x6x7, 4x6x1, 3x6x2, and 2x2x4.  These are to produce one mini horse sculpture with parts to assemble, as well as my flat medallion sculpted above, and another larger medallion I'm molding of another artist's work whom I've purchased ceramic casting rights from.  I know I just need enough rubber to fill in approximately this much space per each mold and I can pour plaster for extra outer supports to conserve on the amount of rubber I need.  If I multiply the three measurements for each approximated mold I get totals like 96 for the first one, 48 for the second one, 126, 24, 36, and 16 which I then add up to get a total of 346.

What does that mean?  It means that my total volume I plan to fill with rubber (or plaster) is roughly 346 cubic inches.  Then I go pull up a handy converter online doing an internet search with the keywords 'cubic inches to gallons converter' and presto!  Up it pops. (I use online converters all the time for mm to inches, oz to grams, whatever).  Turns out that 346 cubic inches equals just a hair less than 1.5 gallons in liquid.  Yay!   This kind of information can tell me about how about much plaster I will end up using for making all six molds, AND also tell me how much rubber because I'm making the rubber masters for plaster mold production.  Therefor since my choices are ordering the rubber in pints, gallons, or 5 gallon sizes, I'll order 2 gallons.  I'll possibly end up with a bit more than I actually need but that's way better than cutting it too close and not having enough to pour all six.  For plaster your voids may be large enough to measure, multiply, then deduct from your total volume.

Other tips - while this won't help with pouring your plaster molds, you can cut up old rubber molds and use them to help fill in larger volume areas during your pour.  This can also keep your rubber cost down.  So if I cut up a few molds into little cubes I can toss that into a 5 gallon bucket until I'm ready to pour my rubber.  Ahead of time I can figure out about how much of the recycled rubber I'll be using and then I can downsize my order perhaps to only 1 gallon instead of 2!  That could save me anywhere from $100 to $250 per gallon I don't have to order in rubber!  Mold Max 40 runs $114 per gallon plus shipping so it's not nearly as costly as some rubbers.

I eyeball my left over pottery plaster.  I have plenty to mix up a couple of gallons of plaster but with the expectation that I'll have some wastage every time I mix, it may be cutting it close.  Besides, I'll be wanting to pour multiple production  molds of each project anyway.  So to be safe I'll pick up another bag of plaster too.

See?  Simple, basic math.  It can be so  helpful!   

Edited February 22 by Hyn Patty

[Hulk]

Spreadsheet maybe?
I use one for updating my Wordle average each day, another for the running list of used words.
...another for bike miles and bike related metrics.
Another for bookkeeping, with a page/tab for each category.
Several with pottery related stuff. 

They can be handy for repetitive maths.
I'd learned to appreciate Excel through work - so handy for analysis (protocol, messaging, transcription...) as well as repeat stuff (students' grades, energy calcs...)

I like the recycle idea!
We've thrown rocks and cement chunks into fence post holes to save on concrete...

[Hyn Patty]

I can't claim credit for the recycling rubber idea.  I got it from the Tiki Technical videos where he's using Rebound 40 which costs about twice as much as the Mold Max 40 I'm using.  But he's also doing MUCH higher production numbers on his mugs than I do with my equine editions.  

I don't bother with a spread sheet for simple math I can do in a couple of minutes, though people can!  But I am a big fan of using spread sheets for more complicated tracking.  Like for every edition I release I use spread sheets to track what's been cast and what # of the edition I'm on, how many I am getting out of a mold and which mold # I'm on, when a piece has been cleaned, fired, packed, shipping paid or pending,  shipping labels made or not, client name & address for orders, what/how/when they paid their deposit, any monthly payments they are making, special directions for alternate shipping destinations -like if they are in Germany, I'm not set up to ship there so pieces get sent to an intermediary who can forward them on for the client, my total costs per piece, special requests by clients are noted and separate pages set up for those if needed, and so forth.  I suppose I could add the math parts to my already existing note sections just to track it.

Running my LLC is a lot of work so there is a lot to track including taxes, quarterly payments, property tax, etc.  Also lots of notes on the usual stuff like testing new glazes, mixing or overlaying of various brands of over glazes like combining enamels with china paints, how glazes change the look of various underglazes, notes on new slip batches I've made, whatever.  As I get older and my memory less reliable I am having to go back and check my notes more often.  Like to remind myself ... mixing cadmium based overglazes with non-cadmium colors is not a good idea!  

 

[Denice]

Hyn  I love the lego's  being used for the walls of your mold.   The old shoe boxes I use don't make a very pretty mold on the outside.   I gave away my son's lego's to a child with learning disabilities.  I gave him all of my son's educational toy's,  I was saving them in case I got grandchildren.    That isn't going to happen so I found a new home for them.  I never new the back end of a horse could be so beautiful.  Congratulation  great work.    Denice

[Hyn Patty]

Hahaha, some of these are my husband's Legos from when  he was a kid, some are my son's, and some I ordered el-cheapo online.  Sold, gave away, or donated anything that wasn't the basic block shapes or sticks.  They aren't really practical for larger mold boxes but they work great for little stuff!

I know, right??  I did the back end of a horse for poots and giggles but turns out I really like the elegance of it! 

Edited February 23 by Hyn Patty

[Hyn Patty]

Oh my Gerd.  I -FINALLY- managed to locate pottery plaster.  What a PITA.  I usually can get it locally for about $20 per 50 lb bag if I pick it up at Highwater Clays in Asheville (a long drive for me but worth it).  But of course they have been out of it and backordered for bloody ever due to ongoing COVID foo backups.  So I finally broke down and called all around until I found Kruger Pottery just got in a load.  Whew!  My big event is in July and I have clayed up so many molds that I didn't dare order rubber until I could be sure I could also get more plaster.   The $100 shipping on two bags hurts but at least it's on it's way!  Hopefully Highwater will eventually get some in and I can get a few more bags yet but without the shipping charges.  Fun, fun.

I did mention that this thread might be better placed in the mold making and slip casting forum... 

[Hyn Patty]

Been a while since I posted about this piece.  I have been really busy with so many other things.  BUT I did get a rubber master cast made of my medallion.  I made it using platinum silicone I managed to get on super sale at a much lower price per gallon than usual.  Which is good because I needed a few gallons for my projects though the shore hardness wasn't quite as high as I usually use and that has caused me some issues with the Porthos mold reproduction.

Anyway, it's not being an issue with the small medallions I am reproducing.  Here is the Cob medallion's rubber master and the first production plaster poured.  Once I've cleaned it up and it's cured, the plaster is ready to use if I want to pour open mold porcelain pieces or I can make a mold back with pour hold for a little thicker pieces, or pieces with backgrounds if I want to go back and carve into the plaster while it's still soft.  So many possibilities!  Anyway, this is the time for me to go back in and redetail anything I want to touch up in my plaster while it's still soft and hasn't fully cured and hardened up fully yet.

But one of the things I am going to do is clean my rubber masters very well, then box them up again and pour ANOTHER layer of platinum silicone over top, to produce a negative.  Once that sets up I'll have a food safe rubber mold I can use for ... *drum roll please* ... casting CHOCOLATES!  OK, maybe I'm just a little bit of a nut.  But why restrict my mold making for only ceramics?  What fun to do your own designs,  your own molds for ceramics, /and/ get to eat your cake too!  So, I can hardly wait to see this boy cast in white and dark chocolate just for fun!  I'll be giving away some of the porcelain medallions at up coming shows as awards this summer but why not /also/ give away some chocolate medallions for awards too?!  Hahahaha!

Maybe I get a little carried away.

Edited April 24 by Hyn Patty

[Jeff Longtin]

Years ago I made plaster molds for a customer who then used my molds to make clay tiles. The clay tiles would be fired and then used as COOKIE molds. (The tiles had a slight relief that would make for very interesting cookies.)

The process, I was told, was to lightly oil the fired tile and then place a little bit of cookie dough on it. Put both in the oven and bake for the normal amount of time. As the dough softened, and baked it, would lay down on the tile and take the relief. Then, once the tile cooled enough, the cookie would lift off the tile easily.

I've often thought of making promotional cookies, for studio events, but haven't taken the time to do it yet. 

Making chocolates sounds fun but challenging. Let us know how it turns out.

[Hyn Patty]

Final photo!  Got my old Seeley's porcelain slip all reconstituted beautifully.  Here are the medallions poured and cleaned up and bisque fired  for the show awards.  The three smallest are intended to be jewelry pendants.  All of these can be kept as porcelain bisques or custom glazed at some later time.  Now back to casting more of them for my event in July!