Tutorial: Detail Casting Plaster

This post is a note to myself in case I forget the ‘recipe’ I’ve been using for detail casting in Ultracal 30. This may seem pedantic but I hate it when I come back 6 months later and have forgotten how I achieved a certain effect or mastered a certain technique.

If you’re plaster casting it might be useful to you as this is how I go about casting 15mm detail pieces with almost no air bubbles. I reject around 1 in 10 pieces cast because of problems with bubbles.

1. Preparing molds for casting:

1. Mix a capful of Finish rinse aid ‘anti-spotting’ agent into a 1.5L soft drink bottle of water.
2. Fill a container that’s large enough to hold your molds with this mixture.
3. Sink your molds into the container so they’re completely submerged.
4. The above steps are essentially the Hirst Arts ‘wet water’ technique.
5. Take a hog bristle art brush and run it through the molds while they’re submerged to dislodge all the air bubbles you can see trapped in the mold details. I use hog bristle brushes because the stiffer bristles help remove bubbles.
6. Remove the molds you’re about to use from the container, pouring out most but not all of the water in the mold. Leave the molds maybe 1/3 full of water depending on where the most detailed part of the mold are. The idea here is you want the mold detail to remain underwater so no new air bubbles can form.

2. Mixing plaster for casting:

I’m lazy so never bothering measuring anything. I find I can mix Ultracal 30 and Hydrostone successfully ‘by eye’ now. It’s really more about achieving a certain consistency of mix than exact measurements imho.

1. Pour a suitable amount of plaster into a container and add the same Finish/water mix to the plaster that you used above.
2. Thoroughly mix the plaster until it’s a thick but still liquid consistency – very like a thin pancake batter. Don’t concern yourself with bubbles too much at this point.
3. After mixing, give the bubbles a few seconds to rise to the surface then ‘dust’ a little more plaster over the top. This will kill the surface bubbles and thicken the mix more. Carefully stir this additional plaster in…avoiding air bubbles as much as possible.
4. Leave the plaster mix to ‘slake’ a for a couple of minutes before you move to the next step.

3. Pouring plaster for casting:

1. Before pouring make absolutely sure the prepared molds you’ve removed from the ‘wet water’ mix above are on a flat surface with nothing trapped underneath them. This is essential for the CD cover method (detailed below) to work properly.
2. Pour plaster into the mold from a height of 5-8cm and as a thin, unbroken stream. This will cause any remaining bubbles in the mix to pop before they enter the mold. Don’t completely fill the molds but cover any detail in the mold with plaster. Recall that the molds are still at least 1/3 full of water. The heavier plaster will initially mix with this water but will eventually settle to the bottom of the molds, displacing the water towards the open face.
3. Take your wet hog’s bristle brush any vigorously run it through the plaster in the mold. You’re trying to dislodge any remaining bubbles in the mold details before pouring the rest of the plaster. Of course plaster is opaque (unlike most two part resins) so you can’t see what you’re doing, but you should see some additional bubbles rising to the surface.
4. Once you’ve finished with the brush make sure you give it a good rinse – otherwise you’ll be throwing it away once the plaster sets.
5. Carefully pour the remaining plaster into the molds, overfilling the molds a little.
6. Take a long, heavy object (I use a steel ruler) and give each outside wall of the mold several solid taps. I find this tapping is just as effective as the ‘pound boards’ or ‘vibrating tables’ people swear by on the Hirst Arts forum. This should release any final bubbles lurking in the mold details.
7. Let the plaster settle into the molds for a couple of minutes until you can see a surface layer of clear water forming in the open face.
8. The open face of the mold may contain some very small bubbles underneath the thin layer of clear water. You may wish to ignore these as they’ll be in the hidden face of whatever you’re casting. However I find these tiny bubbles can be popped by simply blowing air across the mold at about 45 degrees.
9. Take the clear cover of a CD jewel case and slowly smooth it over the mold from one edge and direction. This will press the excess water and plaster out over the edges of the mold. You can see what’s happening through the clear CD cover. Smooth the CD cover down over the mold until it completely covers the mold and all excess plaster has gone. If you find that air starts to leak back in underneath the CD cover and forming bubbles then your mold isn’t on a flat surface, your CD cover is warped or you under filled the molds while pouring. This particular step may take some practice to get working consistently.
10. Once the CD cover is placed over the mold I usually slide it around in a few small circles to spread plaster out over the CD cover a little more. This helps the plaster form a hydraulic seal between the CD cover and the mold and stops air from sneaking back in.

4. Demolding cast pieces:

I generally leave the CD cover on while the plaster is setting however this tends to trap water in the open face of the mold. This simply means you should expect demolded pieces to still be quite wet, despite the fact they’re fully set. You can remove the cover earlier for drier pieces but beware of adding unwanted texture to the open face when doing so.

Ultracal 30 takes around 40 minutes to set before it can be safely de-molded. If you’re casting fine detail I’d leave it for up to an hour before attempting to remove pieces from the mold.

Hydrostone seems to set faster and can usually be demolded in around 20 minutes (or half the time of Ultracal 30). Unfortunately Hydrostone is more expensive for me locally so I always use Ultracal 30.

Both these plasters are very hard once set, however I have noticed that the above technique can reduce the strength of the set plaster slightly. Particularly in the open face of the mold where plaster and water have mixed freely while the poured plaster settles. However a light sand of the ‘underside’ of cast pieces will remove any loose or patchy plaster.