Polyhedron- I’ve self taught myself slip casting over the past month and have learned the hard way that you MUST have a wooden or steel divider. The last thing that you want after all the effort that you’ve put in meticulous preparing your piece to be molded is watching your plaster pour on the ground once immediately breaking through your divider. It will ruin your day and make a big mess for you to clean up. Your divider has to basically be air tight so no plaster will leak through. Close up alol corners with clay to seal your frame. Well worth the preparation!
Although unadvised, i used normal dish washer soap as a release agent but I brushed on multiple layers to make sure my object wouldn’t get stuck.
Finally, make sure that your plaster is properly mixed or it will not dry and you will be left with yet again, another big mess.