First experiments with casting...Part 2

So its now the morning, and the plaster appears to have set, so i set about popping the plaster from its mould... And hit on the first problem! It would seem that both of the mixes that i used were slightly the wrong consistency.

The thicker consistency plaster doesnt look like it set properly, whilst the thinner plaster had an odd "oily" texture to it (it felt wet, but in a very odd way - the same way that olive oil feels!). Here is a picture of the two plasters still in the mould

You can clearly see how the thicker plaster has become lumpy. The thinner mix looked a lot better, just some small amounts of shrinkage around the edges.

I flexed the mould slightly to release the plaster and immediately found a problem with the thinner plaster - it cracked!

After popping out the thinner plaster, i then moved on to the thicker mixed wall - and again it cracked. I was left with two broken walls!

After cleaning up the thicker wall (the thinner one didnt seem to need it), I was left with the four major sections. As a first go at casting, they are not too bad. I dont think they are good enough for the main walls for the diorama tho, Im not sure that they have the structural integrity that i need, but not to worry they will be recycled for use as small parts of masonry or possibly part of the upper level!
So now i'm back off in to the kitchen to mix up some more plaster and retry my moulds... More updates to follow


Chicago Terrain Factory said...

Are you used standard plaster of paris? PoP has such a low strength value, I don't think a 2mm casting is ever going to hold up. You'll either need to double the thickness, or look for a stronger plaster. If you don't want to buy a 25lb bag of hydrostone, try Durham's water putty from Ace Hardware. Durham's is a bit expensive in bulk but for a small project - its priced right.

Paul Chana said...

The plaster im using was sold to me as being suitable for casting these walls by the art shop - aparently its basically the same as hydrocal... but i have so little experience with plasters i'm not 100% sure what the difference is - The second sets of plaster that i've cast have come out much better. these ones didnt crack and appear to have dried a lot quicker. I'll have a hunt around and see if i can find the hydrostone at anywhere nearby - if i use that will it be okay for the thin casting im doing?

Chicago Terrain Factory said...

I've been burned by art stores before when it comes to gaming projects. See if you can locate a MSDS sheet for the product & find out what the PSI rating is. PoP is around 2K PSI. Hydrostone/HydroCal/UltraCal30 is in the 6K to 10K range. Dental plasters move into the 12K to 18K range.

Evernevermore said...

You might also improve the strength a bit by including some sort of matrix into the plaster. The same idea behind carbon fiber or paper mache - find some sort of thin material you can suspend in the plaster. For example maybe window screen or gauze will work, provided you can keep it in between the outer walls of the mold.