Making my first slipcast mold - Advice appreciated

[Ugly Mug Pottery]

Hi All, 

Before I started to venture down my first slipcasting mold project, thought I would gather the braintrust for some tips and potential corrections before I make a mess. I've never done this but I have gathered what I believe is the substantial knowledge to execute but any tips are greatly appreciated. I plan to utilize plasticine clay as a handle shaped as what I desire a finished handle, just 10% larger (to accommodate shrinkage of desired final piece size)

I believe the best way to go about this is to suspend the handle sideways inside the box of wood by wire, using wire as a structural piece inside the plasticine clay. Thicker house electrical structure wire uptop but thin gauge for the actual suspension. The tricky part will be keeping it level, then pour up to half of the subject to make the first mold. Once it dries, add release divets, snip the wire, cover and dress up, apply Murphy's Oil Soap and pour side No. 2.

Logic says the handle design must be 100% symmetrical or else final slip-poured pieces won't release from the molds well and having a true high point in the middle followed by a downslope in handle shape is probably the ticket when the handle lays sideways? Exact straight edges I would only guess are a nightmare to remove from the mold after you've poured the slip. I figured I'd snip the wire suspending it and cover after the first side dries, then pour the second side. 

I see most utilize clay to seal the edges. Thinking I would do the same unless there is a better way around. I thought about hot glueing. I plan to utilize some sort of ramp to pour the plaster slowly to lessen the bubble trapping. In an ideal world I actually design two handles to pour at the same time lined up junction point to junction point but also not trying to overwhelm myself for this first time also. Questions I have...

Q1. Do you prep your wood box before beginning the first pour of plaster? I plan to use Murphy's Oil Soap  I see a lot of this mold release? Would Murphy's be a fine agent for this? 

Q2. I am stuck on removal of bubbles post plaster pour. Just tap the whole thing as much as I can? Do it on a dryer while running? I considered building some sort of stand that a blender underneath running would vibrate the whole box. 

Q3. Lastly, whats the trick on plaster mixing, ratio, how it's mixed in? I figured I'd follow what seems to be the typical 100/70 water-to-plaster ratio and would sift in the plaster and mix in by hand, however, I read agitation of the plaster is actually a good thing for increasing the durability of a better-bonded plaster at finished step? Does this speed up the chemical process by creating friction for it? 

All thoughts in advance appreciated. Hope this all makes sense. Any tips or tricks appreciated! 


 

[neilestrick]

Murphy's will work okay on the wood. The smoother the wood is the better, though, so the plaster doesn't have anything to grip on.

Tapping a bunch works fine. If you were making lots of mold then a vibrating system would be  good, but for the occasional one-off you'll be fine.

https://ceramicartsnetwork.org/daily/article/Plaster-Mixing-101-How-to-Mix-Plaster-for-Ceramic-Molds

 

[PeterH]

1 hour ago, Ugly Mug Pottery said:

The tricky part will be keeping it level, then pour up to half of the subject to make the first mold

I think that you will find this difficult. It's more usual to use some sort of clay "build-up" to keep the plaster under control, as in

PS A handle is nice and thin so there is no difficulty using a solid clay build-up.  For larger objects you may want to minimise the amount of clay used for the build-up.
e.g. https://youtu.be/pXQgi6vW22I?t=698

To state the obvious: bits of plaster can explode in the kiln, so keep any clay used for mould-making well away from the clay you use for making pots. 

Edited February 19 by PeterH

[Ugly Mug Pottery]

26 minutes ago, PeterH said:

I think that you will find this difficult. It's more usual to use some sort of clay "build-up" to keep the plaster under control, as in

PS A handle is nice and thin so there is no difficulty using a solid clay build-up.  For larger objects you may want to minimise the amount of clay used for the build-up.
e.g. https://youtu.be/pXQgi6vW22I?t=698

To state the obvious: bits of plaster can explode in the kiln, so keep any clay used for mould-making well away from the clay you use for making pots. 

You think this is doable with a handle made out of plasticine clay? This feels like it requires a fully finished handle. 

[Jeff Longtin]

Hello Ugly Mug, welcome to the Forum.

The video Peter posted does a good job of showing the process. Creating a bed is the best "first step" in the process. Like the video I choose to make a bed using foamcore. (Its cheap and flat and clean.) I use clay to seal the gap between the shape and the foamcore.

Every so often I coat my plywood pouring boxes with Johnson Paste wax. It won't prevent the plaster from sticking but it makes it easier to separate the plaster from the wood if it does. (If the plaster does stick to the wood just tap it with a hammer to dislodge it.)

I don't tap the bucket or the box. I gently create a wave in the plaster mix and let the bubbles rise that way. 

I like to hold a jiffy mixer in my hand and mix plaster that way. Using just my hand doesn't seem to agitate it enough.  

[Jeff Longtin]

Plasticene vs plaster forms - You can make plaster molds from plasticene forms but the heat of the setting plaster may deform the plasticene original. That really sucks when the client spent months creating the form.

After you pour the first half of the plaster mold you will need to apply Murphy's Oil Soap to the surface to seal it. Brushing around a soft plasticene form takes a delicate hand. The first few times I did it I left brush marks on the plasticene form. Working from a plaster form removes those challenges.

[PeterH]

20 minutes ago, Ugly Mug Pottery said:

You think this is doable with a handle made out of plasticine clay? This feels like it requires a fully finished handle. 

Yes.

Of course you are going to have a seam in the middle of the handle which may leave some sort of casting mark in the handles  which you will need to make-good.

PS Interesting but of low relevance: a thread that started out with a question on cast handles and digressed big-time.

 

[Chilly]

If you are mixing your plaster by hand.....

Line a bucket with a plastic bag and mix the plaster in that.  When you have poured the plaster out you can just throw the bag and any leftover bits of plaster.  And not have to mess about cleaning off the dried out plaster.  This was an absolute game-changer for me.

[Hyn Patty]

Suspending the handle with wires is a far more complicated way to do it and just using more oil clay to lay in a bed complete with keys.  Pour one side in plaster, then flip it all over, remove the oil clay bed from the handle, then pour the plaster on the second side.  Very easy.

That said, I'd suggest you sculpt the handle you want and get us a photograph both of the handle at a few angles and how you think you want to set it up in your mold box.  Much easier for folk to help you if we have visuals.  Otherwise you already got lots of great advice here.