Powders

Sugar

http://www.sucrose.com/learn.html

http://www.dominosugar.com/

http://en.wikipedia.org/wiki/Sugar

Flour

http://www.nabim.org.uk/flourproduction.asp

http://www.flourandgrain.com/flourandgrainfacts.asp

http://en.wikipedia.org/wiki/Flour

Baking Soda

http://www.armhammer.com/

http://www.bakingsodabook.co.uk/

http://www.recipes4us.co.uk/Specials%20and%20Holidays/Baking%20Powder%20and%20Soda%20Origin%20Uses%20Recipes.htm

Cornstarch

http://www.answers.com/topic/cornstarch

http://www.argostarch.com/index.htm

Salt

http://www.saltinstitute.org/4.html

http://en.wikipedia.org/wiki/Salt

http://www.mortonsalt.com/

Plaster of Paris

http://www.artmolds.com/ali/history_plaster.html

http://en.wikipedia.org/wiki/Plaster

http://www.answers.com/topic/plaster-1