Re[2]: ATM - Casting Aluminum

[Dwight Elvey <elvey@hal.com>]

"Michael Perata" <michaelp@p-solutions.com> wrote:
> 
> You partly blew up the concrete along with the AL. Concrete, dry for thirty
> years, still contains trapped water. You could have experience the 1" hole
> without the AL. BE CAREFUL!!

Hi
 Even oven dried concrete has water in it that will come
out at the temperatures used to melt aluminum. This is
how concrete cement is made. They have a high temperature
rolling furnace that heats the gypsum until the water
comes out of chemical bond. When you add water, it
recombines and makes cement. This is a fully reversible
process. Heat it again and it turns to powder.
Fused cement doesn't have enough relief to let the water
out and tends to blow up. Most of the plasters we use
to make tools out of will do the same. They use special
clays in refractory cements instead of gypsum. These
can withstand direct torch flames without blowing up.
Next time use fire bricks.
Dwight