Can you use plasticine clay for model to be slip cast

[lta]

I used plasticine to create a figurative mug and want to then make a mold and slip cast( or try to, lol) I have read conflicting information about using plasticine for this purpose. Ultimately I did choose to use it over wet clay for the control over detail, reuse and no dry out. Can anyone offer some advice about how to move forward and or if there is anyone in the Boston area that teaches mold making? I have looked high and low with no luck. Thankyou

[High Bridge Pottery]

There's no problem using a plasticine form to cast plaster molds from, it could be a bit tricky to block off areas for multi-part molds but some normal clay will probably remove pretty well when you come to pour the next section.

I did read some plasticine can mess with silicone curing but nothing about plaster.  My only advice is to get a soft natural hair brush and paint on a thin layer before you do the main pour, that's the only way I found to get rid of most/all air bubbles on the surface. Tried different surfactants and different soaking times and mixing times and they all seemed to trap bubbles on the surface no matter what.

[lta]

4 hours ago, High Bridge Pottery said:

My only advice is to get a soft natural hair brush and paint on a thin layer

? Hi, thank you for the response -paint on a thin layer of ? plaster? on the plasticine? This is my first foray into casting or I said before trying to! I have seen people use murphy 's wood oli soap  as a release?

[High Bridge Pottery]

Sorry I probably didn't explain it the best. I don't mean paint a layer and let it set and then do the mold.

Once I have the mold box set up and ready to cast in plaster I will weigh out my plaster and cold water in the right ratio. Add the plaster to the water and let it sit for 1-2 min then gently mix with my hand for 2-3 minutes until I feel it slightly thicken. Now that I am ready to pour the plaster into the box I will use a brush dipped into the plaster and paint onto the form to get into all the detail and then pour the plaster into the box to cast the form. I found if I don't brush the plaster onto the form and just pour the plaster into the box I end up with bubbles on the casting surface.

 

I got the idea from this video, he is not using a box to cast the form and lets the plaster thicken up after brushing a lot more than I would to pour the mold but the brushing idea is the same. Later on in the video when he is pouring the plaster head you can see the kind of consistency I would pour the mold at after brushing it onto the form to capture all the detail.

 

Edited February 23, 2023 by High Bridge Pottery

[lta]

Thank you for the video, i think I may have watched this one before! It's a great video but never thought to apply the technique to what I am doing now, makes sense though. I'm just afraid to take the plunge because it could easily be a three-piece mold because of the undercuts and detail. But actually after watching this I may take a break and do something more straightforward and cast that first just to get my feet wet. Thanks for the imput!

[Chilly]

On 2/23/2023 at 8:33 PM, lta said:

do something more straightforward and cast that first just to get my feet wet

Pick something really easy, like a piece of fruit - apple, orange,  banana -  or an existing cup or mug.  (Bear in mind that anything cast will end up smaller than the original.)  That will get you thinking of undercuts and how to section off.  

[Jeff Longtin]

Hello Ida,

Using plasticine isn't bad but some varieties soften with heat, from the setting plaster, and this can make it difficult to pour the second mold part.

If you want to post a picture I can offer more thoughts. 

 

[lta]

Thanks to chilly for the response, I will give it try that is something simple though I am used to being in over my head, lol.

Hello Jeff,

I have seen videos about that issue, the hardness of the plasticine maybe effected by the heat generated during the curing process of the plaster. I am using a med hardness for this piece which seems firm but that does give me pause in that I had planned to do a simple piece first just to get started and (duh) why not do two in one and test the hardness /potential warpedness of the various plasticines at the same time test my plaster mixing skills.

I'm always getting ahead of myself, lol

[Hyn Patty]

You already got great reponses here so I  have little to add!  Most of my originals are sculpted in oil clay and initially I did all of my plaster molds directly from them.   If you are careful to use oil clays that are sulfur free you can also avoid most issues with rubber conflicts.   As already noted however, if you are pouring large plaster sections the heat can cause your oil clay originals to melt and deform just like with wax originals.

Once you get some practice and confidence, and you move into higher end pieces you invested a lot of time into sculpting, you may also wish to consider casting rubber masters.  If you make a rubber master of your original, and rubber master pieces of your mold parts you planned to use in plaster, then you can simply recast more plaster production molds over and over again from your rubber masters.  Plaster molds, especially those with very fine detail such as my work, are only good for about ten to twenty casts before the molds will begin to show wear.  The more detailed and complicated the mold the faster you will see problems, especially if you do not clean and store your plaster molds properly after each use.  Simple, low detail molds will last a lot longer.

VanTiki posts wonderful Technical Tuesday Youtube videos that may be hugely helpful to you to learn mold making.  There are also some great blog posts out there if you do internet searches.  Here's a great video on rubber master molds for plaster slip casting right here.  He posts a lot of videos on regular plaster mold making so look him up!

 

[Hyn Patty]

Here is another VanTiki video, this one only focusing on making a plaster mold.  This one also shows you about how to make your mold walls using wood (usually a simple box you can slide to adjust sizes, this one is more of a chair form.  You can also make your molds using Lego blocks instead of sealed wood which is my choice when I'm making smaller molds to pour.

 

 

Edited March 1, 2023 by Hyn Patty

[Hyn Patty]

On 2/27/2023 at 4:27 PM, lta said:

Thanks to chilly for the response, I will give it try that is something simple though I am used to being in over my head, lol.

Just want to say BRAVO!  Have no fear and  jump right in.  Even if you ruin it, you'll learn from it.  Biggest problem I run into teaching is that people are /afraid/ of failure.  I've killed countless plants learning how to grow and propagate them, and I've ruined many a painting and sculpture, mold, and casting.  But I learn best by DOING.  So jump right in and don't be afraid.  You got this!  Start simple, take small steps and if you need help, that's what we are here for!

[lta]

Thanks so much! I stumbled onto his videos a few days back and they are great but I think I will take your advice and just put the laptop down!! 

[Jeff Longtin]

I'll pick up on what Hyn Patty says: some of us started making molds before the internet and we really only learned from our mistakes. In theory plaster is inert and should behave the way we expect it to.  Unfortunately that is not always the case and you need to improvise. That's when new ideas are born and new methods created. Continue your trials and feel free to post questions when they arise.