OblivionFall Posted March 16, 2015 Posted March 16, 2015 Hi there, I'm very set on making Thermite Putty like in this Instructable: http://www.instructables.com/id/Cast-Thermite/ I have a few Questions. Why use Plaster of Paris instead of a binder like Dextrin? Would making the ratio of Red Iron Oxide to Aluminum Powder 76.3 : 27.3 be better than the 3 to 2 shown in the Instructable? Also, because I feel that the Plaster of Paris would dilute the Thermite, could I use less P of P or some other binder completely to increase the effectiveness of the Thermite?
Ubehage Posted March 16, 2015 Posted March 16, 2015 Also, because I feel that the Plaster of Paris would dilute the Thermite, could I use less P of P or some other binder completely to increase the effectiveness of the Thermite? Actually, you want to dillute the Thermite, to cast it properly.Using less plaster, might lead to a dry clay-like consistency, that tends to crack easily, while working with it and while drying. If you are worried about making the Thermite less agressive by mixing in non-flammable binders, I would suggest NC Lacquer.You get a really nice, smooth putty. And you almost can't add too much.
FlaMtnBkr Posted March 16, 2015 Posted March 16, 2015 Actually, plaster of paris is a pretty good high temp oxidizer. There is a chance it was used for a reason and not just randomly chosen, 1
Arthur Posted March 16, 2015 Posted March 16, 2015 Calcium sulphate is a good oxidiser for hot energetic reactions. It's used in underwater flares.
BurritoBandito Posted March 17, 2015 Posted March 17, 2015 To add to what FlaMtnBkr and Arthur said, there are some formulas in which CaSO4 is the ONLY oxidizer. http://www.nakka-rocketry.net/thermites.html#TH11 K. Calcium Sulfate & aluminum Calcium sulfate, CaSO4, is readily available in the form of plaster of paris, blackboard chalk*, or gypsum. As plaster of paris, it exists as the hemihydrated state with ½ molecule of water, CaSO4.½H2O, or as chalk or gypsum, in the dihydrate state, CaSO4.2H2O. In the anhydrous state, it is used as a dessicant. Before using in a thermite mixture, the material should be dried in an oven at 175oC. (350oF) for an hour or more to reduce the moisture content to the hemihydrated form.*Blackboard chalk may be made from either calcium sulfate or calcium carbonate. To confirm a given chalk is calcium sulfate, drop a piece in strong acid (e.g. muriatic). If it fizzes, the product is calcium carbonate. If there is no (or minimal) fizzing, the product is calcium sulfate.The assumed balanced equation is: CaSO4 + 2 Al -> Al2O3 + CaO + S The stoichiometric mass ratio of oxide to metal is 2.43 :1. For the hemihydrate form, the mass ratio is adjusted to account for the ½ molecule of water: The mass ratio of hemihydrate oxide to metal is 2.6 :1. For a 10 gram batch:CaSO4.½H2O 7.2 gAl 2.8 g Observations: Quite difficult to ignite, requires magnesium ribbon to initiate. Burns very rapidly with a bright flash. Could not be ignited with a Mini bulb igniter.
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