Problems with plaster casts

[MickeyM79]

Hi everyone. This is my first post on here. I'm from the UK and use a gypsum plaster called Crytacal R. I. Not sure how many people are familiar with it but it's one of the strongest and better quality plasters available.

I've worked with this plaster for some time just learning the trade of sculpturing with the intention of selling some designs but recently I've been having an issue that I can't resolve where some flat surface areas are concerned. Mainly the base parts and some flat sections. I've noticed that these straight flat surfaces seem to bow or have a slight bend in them so they don't sit flat on the surface whereas when I originally removed from its mold it was flat. The actual mold doesn't have these bends so originally I was wondering if it was because I was removing the plaster too quickly from mold pretty much as soon as it had hardened. However I've always removed casts pretty quick and never had an issue with straight edges or surfaces before.

The plaster is only a few months old so there isn't an issue with the age if it. Can anyone help or give some advice.

Many thanks in advance.

 

[joberg]

I'm sure you've respected the instructions to the letter (water/gypsum)...any changes in temperature or humidity lately?

 

[MickeyM79]

Yes this particular mix is 35:100 (water/gypsum). I do use re-use plastic mixing containers and on occasions havnt been able to get out all the hard plaster in the bottom from a previous mix, however I can't see that being the main reason I'm having this problem.

I do get good casts but every so often get some with the problems described. What I want is to get a good consistency which seems to be the issue at the moment and I'm wasting alot of plaster.

 

[basementdweller]

I am thinking you are getting uneven curing on account of temperature differences between the sides. Get an oven rack and prop it up on the sides and then place your sculpt in the middle so air can flow freely on all sides. Cold water takes longer for the reaction to start and thus for the plaster to set so water temp might also play a role in consistency.