Leaky Mold Remedies?

[Ben xyz]

I have a fairly large plaster mold (approx 15"x20") that I've tried sealing its seams with clay pushed in from the outside before pouring in slip, but leaking persists. Only  after it initially leaks and dries a bit, can I continue (after cleaning up the previous leaking mess).
1. I've read here that some have used slip from the outside along the seam. With the initial pressure of 3 gallons of slip poured into the mold, I guess I'm questioning the effectiveness of this method, though will try it of course (assuming I use a turkey baster along the seam?) to see if it works.
2. Was considering waterproof gorilla tape, though the adhesion to plaster may not be sufficient.    
3. Non-hardening plastilina modeling clay (guessing oil-base) might be a possibility to plug up the seams?  
Experience? Thanks!

 

 

 

[Shane]

Hi Ben,

I've poured a lot of large molds with bad leaks and I've never had to do more than the following,

1. Use 1 (or more if required) proper clamping mold straps to make sure the mold doesn't move or shift at all when casting it.

2. Use the right consistency of clay to plug up the seams on the outside, fresh soft block clay is about right. This comes with experience but you can feel it going into the seam when done correctly. I rarely do this but if there's a large gap or hole on the inside of the mold that you can reach then push some clay in that as well, obviously use the same clay as you're casting with.

3. Give it 5 or 10 minutes to stiffen a little before casting the mold (depending upon environmental conditions). If you can see cracks it's far too dry.

4. Sometimes the seam will still spring a leak when cast but usually (after finishing the pour to avoid lines) just running your finger along the seam again will reseal it. If not  jam some more clay in the leak.

5. If all the above doesn't work the mold is probably too far gone and should be disposed of.

Hope that helps.

Edited July 15, 2023 by Shane
to clarify

[Jeff Longtin]

Separate the two processes: 1) filling the seams/gaps, and 2) filling the mold.

1) pour a small amount of slip into the mold and use a brush to push the slip into the seams/gaps. Wait a minute or two. Use the brush a second time to push the slip into the seam/gaps. Wait another minute or two. Do the same thing a third time if necessary. Wait another minute or two.

After you see the seam/gaps sufficiently filled THEN go ahead and start filling the mold. Go slowly to make sure your gaps are filled and no slip is leaking out. 

If it looks like you filled the gaps sit back and breathe a sigh of relief.

Right now my purse vase molds are just plaster parts held together with straps and rubber bands. The shape, 8" wide, 8" tall, 3" deep, is really susceptible to blowout if I forget to seal the seams. The above method assures that the slips stays in place and I get a decent casting without a lot of mess or stress.

 

 

[PeterH]

41 minutes ago, Jeff Longtin said:

Separate the two processes: 1) filling the seams/gaps, and 2) filling the mold.

As I find pictures helpful in forming memories, not a dissimilar 2-stage process to that in
Inlaid Slip Casting
https://ceramicartsnetwork.org/pottery-making-illustrated/pottery-making-illustrated-article/Inlaid-Slip-Casting

Edited July 15, 2023 by PeterH

[Ben xyz]

Thanks Shane, Jeff and Peter for relating your experiences - great suggestions! Will try ALL. Determined to make this work. I also need to be vigilant about cleaning out the mold completely in-between castings. Using two straps but may add another from the other direction. Thought perhaps waterproof tape could work or the non-hardening plastilina modeling clay, but maybe all an overkill.

[Ben xyz]

Bingo! This product totally worked for me (pushing it into seams from the outside). Easy breezy. Already had it lying around, so thought I'd give it a try. Sticks well enough to the plaster, but easily peels off after casting, to be used again:
Sargent Art Plastilina Modeling Clay, Non-Hardening

(unable to turn off bold feature above after cutting and pasting product name - not really shouting) 

Edited July 24, 2023 by Ben xyz
Attempting to remove bold feature

[Callie Beller Diesel]

I have seen people use waterproof packing tape to support moulds, in addition to mould straps. The trick is to make sure the tape is wrapped all the way around and stuck to itself, not the plaster. They explained that it was good because it doesn’t stretch. The straps are still necessary as a backup in case the tape snaps under the weight of the slip.