How to make a cavity mold

After having about 9 lbs of ceramic slip leftover from a previous project, I figured I might as well slip-cast until I have none left.  So with some help from the internet and my friend/ studio neighbor, Alex Mac, I made a cavity mold from these cute little house bottles I found at a flea market.


A two-piece cavity mold is ideal for pouring a liquid into a mold that will harden, such as wax, rubber or slip.  The mold has a hole either at the top (for vases, cups, bottles etc.) or the bottom( for statues or objects)
This type of mold is best for slip casting because the plaster absorbs the water from the slip, which allow the casted object to harden into its shape.  Directions for slip casting will be posted soon.

Supplies:
foamcore board or cardboard
x-acto knife
object to cast
clay
vaseline
plaster
duct tape 
ruler

Step 1a- place your object on your board and draw a square or rectangle around it.
* note- the top of the object will be close to the top edge, the other 3 sides should have enough space to make grooves to lock in your mold when you use it.
Step 1b- measure the depth of your mold and cut out flaps (object’s width+ 2 inches) to fold up to make walls

Step 2- score the bottom of your box to fold up the sides and tape it up really well, making sure to tape up your score lines also.

Step 3- take your object and cover it with clay halfway, to where the 2 halves of your mold will come together.  I used ceramic clay, because I had it, but I’m sure sculpey will work just as well.

Step 3a- place you object in the box and fill gaps with clay.  (It doesn’t need to be perfect, but the smoother it is, the better your pieces will fit together in the end. ) after filling in the clay, press your index finger into the clay 2 or 3 times in different places to make shallow craters
Step 3b- (it is not pictured here) but you also want  make a little hill starting at the top of the object, widening out to the wall.  ( reference the picture for step 7)

Step 4- cover your object, clay and walls of the box with vaseline being careful to get every crack and corner.

Step 5a- make the plaster and pour over the object covering it completely, and then some more.  you can also mark your box to where you want the plaster to meet if you think you will be unsure.
Step 5b- Once you poured your plaster into the mold, tap the sides and wiggle it around a bit to get any air bubbles out.
* wait at least 1 hour for the plaster to dry.  i know! be patient.

Step 6- when the plaster is dry, slice the tape and open the box.  it is ok if the cardboard sticks to the plaster.  take out the block of plaster and remove all the clay, leaving the object in the plaster. place the plaster block into the box, object facing up, and tape it back together.

Step 7- fill in any gaps with clay and make another hill at the top of the object. (sorry for this pictures, I had made another mold because the blue house wasn’t as good as the green house, and this mold came out better in the end.)

Step 8- repeat steps 4 and 5

Step 9- wait for the plaster to dry completely at least 1 hour.  BUT i would wait overnight because you can’t use your mold for 24 hours anyway. :)

Step 10- when the plaster is completely dry, slice open the box and carefully separate the two pieces.  remove your object and HAPPY CASTING!

<3 G

 

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One thought on “How to make a cavity mold

  1. Pingback: How to slip-cast « full as a bean

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