An Instructional Video on Sand Casting 

Yes. Only a small amount of burnt surface clay is thrown out.

@@philanle4556 I vent everything.

Hi Debbie, borax is added to help the metal flow better but it doesn't have to be added. No, it isn't a problem to let the flask sit for awhile to cool but I open it quickly to keep the video short.

It cleans the metal of impurities by aiding it in separating dissimilar viscosities while it's molten... If you sprinkle some on top while it's molten, you'll notice it draws up the impurities and then you flash, (remove the flame for a second), and see that the surface tension is changed, and the impurities are drawn to the sides of the crucible for that second it had cooled... It also acts like a flux creating a barrier of gas which prevents the metal from reacting with oxygen in the air and the air feeding the torch if you're using Oxy/Propane. It basically acts as a protective shield and aids in keeping the molten metal viscus. That's to the best of my understanding, but if I'm incorrect, please feel free to let me know. :)

You can either weigh it out first or guess which is what I did. My typical pour is 1 1/2 ounces which works for most jewelry size items.