Easy Plaster of Paris Strobe

[BlacksmithNL]

Dear All,

I've never posted any of my work on this forum, but decided to do so as it definately falls into the realm of amateur pyrotechnics. The formulas I post are flash-type reactions, so if you are not familiar with any of these, please be warned: Easy does not mean safe. Here goes...

 

Being fasinated by pyro-chemistry and particularly the use of easily obtainable, safe chemicals to make pyro, i started fiddling around with a strobe mix made up of Plaster of Paris, Sulfur and MgAl (50/50). The idea for this came from an article by Shimizu where he discribes the strobe mechanism. In this he suggested various chemical pairs for the "flash" reaction and some for the "smoulder/dark" reaction. Two of these pairs caught my attension. BaSO4/Al for the flash- and S/Mg for the smoulder-reaction. Having done some experiments with CaSO4 (Plaster of Paris) and MgAl, I knew that this made a pretty decent flash if the CaSO4 was completely dry, so i wondered if it would be possible to make a strobe mix using MgAl, CaSO4 and S.

I started by writing the reaction equations for CaSO4/MgAl and S/MgAl assuming sulfide products and came up with:

A) CaSO4/MgAl = 63/37 (wt%)

S/MgAl = 61/39 (wt%)

I tested these. The CaSO4 reacted as expected. Flash! The S/MgAl surprised me a bit. This mix "burns" with a flash&pop! Scary stuff and definately not a smoulder reaction, but i decide to continue...

Next I mixed a 1:1 ratio of A and B. As i couldn't use a waterbased binder with the CaSO4, i decided to bind with +10% NC made from ping-pong balls dissolved in acetone. I let it dry and then tried to light a small pellet wearing leather gloves, leather apron, face shield and ear plugs. Nothing happend except when heating for a while in which case it would flash and go out abruptly. Not exatly what i was expecting and after thinking on it for a while, i guessed the problem might be that there is not enough of the "smouldering" reactiong going on to initiate the "flash" reaction.

Next i tried A:B = 1:2. Garbed in my chicken-suite, i tried again. It worked! The mix burnt with beautiful small sparks and then would flash unexpectedly leaving me completely blinded. The flashes were irregular between 10 end 15seconds and would sometimes go out after a few flashes due to the intensity of the reaction. Still seeing black spots everywhere, i cracked open a beer and celebrated my success. It was a good day.

Next i decided to increase the amount of B to A to see if i could get a stable reaction. I tried A:B = 1:3 and A:B = 1:4. The 1:3 mix performed the best with stable flashes around 4sec. The 1:4 mix flashed irregularly between 3 and 5sec throwing of a lot of sparks.

Sticking to the 1:3 mix i decided to add some KNO3 to the mix in an attempt to increase the flash rate. I added +10%wt KNO3 to B and this worked, but the flashes came more irregular and burning longer. At this point my experimentation was interupted by life, so i decided to share what i have hoping to pass the baton.

Currently the best mix is:

S (#70) 47%

MgAl (<45um) 37%

CaSO4 16%

Bind with +10%NC from ping-pong balls

The Plaster of Paris (CaSO4) is for modeling and is super fine. I bake this for 1hr @ 200oC to drive off as much moisture as possible (CaSO4.1/2H2O -> CaSO4). The ingredients are sieve-mixed 3times using a #30 sieve. NC is mixed with Acetone and incorporated into the mix. Acetone is evaporateded during mixing until a putty is obtained that can be made into pellets. Fast drying gives the best results.

Note: 5%NC is probably sufficient to bind this mix, but i add the extra 5% to prevent the absorption of moisture. This seems to work as i've tested pellets left outside and these seem to perform the same as stored ones. I would recommend using a drying box as there is some chance of moisture absorption during the drying process.

I've not made stars or grains from this so have no data on it's performance during flight. It does give a very novel experience as this burns with a beautiful volcano like effect until the flash which is absolutely blinding! :-) The repeat rate is about 0.25Hz (4sec).

I look forward to hear if any of you have done work on this and look forward to any follow-ups on my work.

If you can't be good, be good at it.

U.

P.S. The pellets light quite easily, but if you are making them into stars, i guess a prime would be neccesary. One could also granulate this and add them as micro-stars to a willow-mix. The binder probably needs to be replaced by an alcohol-based binder (shellac/red gum) to prevent water getting into the CaSO4. A chlorine doner (PVC, Parlon) can also be added to the mix to get orange-flashes. I've not done any of this, so have no idea what the resuls will be.

[a_bab]

Nice find.
With MgAl being so fine I don't think there is any room for improvement in making the reaction faster.
But changing the ratios may help.

[BlacksmithNL]

Adding an oxidizer might help to increase the rate. I've tried KNO3. It does make it faster, but at some point the pellet starts to burn. The 1:3 ratio is the best ratio of the mixes i've tested, but to find the optimum i would recommend tryng A:B = 1:2.5; 1:3.0; 1:3.5 and see which performs best. Then sub-devide and try again. If using as a star adding more MgAl might help. Needless to say, there's still some some work to be done.

U.

[Yus]

BlacksmithNL, It is nice job. But you should know that pyrotechnic system or compositions like Me(NO3)2 and/or MeSO4 and AlMg and/or Mg and sulfur are widely used as common strobes or microstrobes in the South America and little in Europe and China. Pyrotechnicans from those countries do not tell exact compositions and for sure I know this from them personally.

[davidh]

What is Me?

[a_bab]

Metal.

[BlacksmithNL]

Yus, I am quite aware that others have made similar compositions, else Shimizu would not write about it. There is nothing new under the sun (apparently), but my formula is the first i know of in the public domain. It's easy and effective. Maybe a bit slow, but quite usable to anyone with minimum resources.

[Yus]

Me = Ca, Sr, Ba. BlacksmithNL, have you video of burning star? Have try to use SrSO4 instead of CaSO4*H2O? SrSO4 and BaSO4 are not so expensive, and do not contain H2O.

[BlacksmithNL]

Yus, I haven't tried SrSO4 of BaSO4, but they should work. Unfortunately the ratio will probably not be the same. I used CaSO4 as it can easily be obtained as a modeling plaster. CaSO4 isn't super hygoscopic, but it does absob water over time, hence the need for NC or other waterproof binder.

[BlacksmithNL]

I have only bad video of the pellets burning. The flashes are so intense that my camera looses all sense, difficult to capture.

[Yus]

We would like to see your good video.

[Mist]

Nice development.

[cevmarauder]

What size did you make your pellets, out of curiosity?