Brushing clay into plaster mold, terrible surface quality of clay positive

[MRBANANA101]

Hey all - I'm attempting to brush melted monster clay into a large plaster mold so that I can do a bit of resculpting on the monster clay positive. I'm able to remove the monster clay positive from the plaster mold mostly successfully (by breaking the plaster mold), but it seems no matter what I do, the surface quality of the clay is always full of little gashes and tiny holes. Even though the clay brushes in very nicely into the plaster mold, the surface quality of the clay always suffers, and I'm assuming that maybe the hot clay against the plaster probably cools too quickly, causing subsequent brush strokes of hot clay not to overlap properly, resulting in gashes and poor quality? If that's the case, is there a method for keeping the plaster mold itself warm? Basically, the question is: Why is it that every time I brush hot melted monster clay into a plaster mold, the clay positive always turns out terrible, full of holes and pits? I've seen Brick In The Yard videos on youtube and they do a very similar process, except they brush the melted clay into a silicone mold instead of a plaster mold, and it turns out wonderfully. What is it about plaster that's causing the terrible surface quality of the clay? Or is there something else I'm missing?

Just a bit of background info. The original positive that the plaster mold is taken from is a very large (taller than 5 ft / 1.5 meters) 3D printed plastic (PLA) figure. It wouldn't work to modify the original plastic figure by hand, as the countless hours involved in sanding and attempting to sculpt PLA at that size can drive a man mad and certainly negates the hassle of making a plaster mold. Because of that, I decided to make a plaster mold and make a clay copy of the original PLA print that I can sculpt and add super fine detail to. I'm having no troubles making the plaster mold, or removing the final clay positive - just that the clay surface quality is terrible.

I've thought of alternative mold materials instead of plaster, like using alginate, but for such a large figure, considering alginate's working time, inability to bond with itself, and short time to cast into, it's been a bit of a disaster, even using alginate retarders. I could make a silicone mold for sure, but the cost of using silicone just to make a one-time use waste mold makes me shiver.

Thanks very much!

 

[zorg]

I kinda agree with your thoughts on the hot clay cooling on the colder plaster, can you heat the molds in an over first?

I think I saw that brick yard VID, didn't they heat the silicon mold first??

I didn't read your entire post it's way too long. Sorry.

 

[Bloop]

Basically, the question is: Why is it that every time I brush hot melted monster clay into a plaster mold, the clay positive always turns out terrible, full of holes and pits? I've seen Brick In The Yard videos on youtube and they do a very similar process, except they brush the melted clay into a silicone mold instead of a plaster mold, and it turns out wonderfully. What is it about plaster that's causing the terrible surface quality of the clay? Or is there something else I'm missing?

I don't have much experience with mold making, and this is just a guess, but I'd say a basic difference is that plaster is porous and water soluble, while silicone isn't. Plaster doesn't permanently set like silicone. If you soak plaster in water, it will absorb the water an errode and disolve. Silicone won't. I assume the clay is interacting with the plaster and pulling moisture out of it, and melding with the plaster mold, causing those imperfections.
At least, these are my assumptions. Someone with more knowledge can probably chime in and correct me if I'm wrong.

 

[MRBANANA101]

Hate to revive such an old topic, but I thought it'd be helpful to update with my findings since then, incase any one else comes across this thread in the future with similar issues. There were two main reasons the surface quality of brushed melted clay into a plaster mold was so poor:

1. I wasn't using any sort of release agent. Honestly, I didn't need anything fancy at all. I use just a bottle of palmolive dish soap brushed liberally into the plaster mold, let dry for a bit, wipe down any excess residue. The clay, once cooled and hardened, releases perfectly from the plaster mold and significantly improves the surface quality of the clay cast.

2. brush strokes should be wide, fast, overlapping. There's a bit of a technique to it, but making sure we fill the plaster mold with very broad brush strokes of clay, overlapping previous strokes, working quickly, all helps to support a cleaner, consistent surface quality. Hot clay blends and adheres much better to more hot clay, so moving too slowly and allowing the clay to cool too much in the plaster mold will hinder the quality of the final cast.

Now, granted, this still won't lead to a perfect clay cast free of any surface blemishes, but it's definitely more than clean enough to only bother with some lighter manual touchup - smoothing the clay, filling in little gaps, etc. Then hitting it with some sand paper and isopropyl myristate if needed.

 

[Mottrex]

Cold mold seal is a great barrier release agent used to paint on plaster of Paris...You can paint it on warm plaster too and it sets quicker, I used it earlier today when processing acrylic dentures, if it releases them clay should be a cinch..