Re: Investment life


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Posted by bpfink on April 02, 19100 at 08:56:45:

In Reply to: Investment life posted by Tony on April 01, 19100 at 10:12:13:

Two interesting questions.

I don't know the life of the mold once invested but it is nearer to as long as your interest to store it.

Plaster and sand is a pretty stable item (witness the older buildings in Europe that used it for their walls) and the wax inside is not going anywhere either. Being so alkaline it doesn't even support other growths to break it down unless forced to under rare studio conditions (warm and wet and while being wrapped in plastic with bacteria or your unfinished lunch setting on top , etc.).

As far as the moisture in it goes. Most folks I know of let it dry out even before burning it out. Some even set them outdoors in the sun (to save fuel I would guess).

I do the reverse.

When ready to burn it out I keep it wet, and even rewet it sometimes. My molds often reach the 1000 to 3000 lb range but even with smaller ones I rewet them if they have been around too long.

It means of course there could be is a longer time needed to burn it out since that may mean there is a good 55 gallons or more of water in a kiln load, but that is okay, and it so far has not appeared to even take longer.

A dried plaster investment is a fairly good insulator when compared to a wet plaster, especially if there is any vermiculite or expanded mica or detergent forced mini air bubbles included.

I like the wet version as that is a better conductor and the whole mold will slowly rise in temperature at a more even rate throughout. This results in a better elimination of flashing that could be caused by mold cracks that the heat stress differences could bring about.

As a test for yourself. Cast up 2 identical molds of small size and keep one wet and let the other air dry first. Fire them and pour them together and see what happens.

The difference is minor but for me was still noticeable.

I also use styrofoam sprues and thus have a way to remove a core entrance into the middle of the piece before the item even goes into the kiln. (First, a red hot welding rod plunged down the length of it as far as it goes, then a 1/2 cup of lacquer thinner poured in to remove the balance and to eat it's way right to the wax inside.)

To rewet a plaster mold, short of setting it in a tub of water (not possible with some of my largest ones) I simply put a plastic gallon jug of water on top and punch a hole in the bottom of it. Then refill it a few times til it no longer seems like a dry mold. Pouring water down the open sprue also gets to the center faster.


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