8 replies to this topic

[Tinderbox]

I mentioned this last night in the Formula section in the Stars thread but thought I'd ask here too. I can't find any discussion on it but that's because I do not know the term for the phenomenon that is: 

A star which during drying, expands and cracks open. Usually in the latter stages of drying.

 

I have been working on a Sodium Nitrate based composition using fairly coarse MgAl. I used 1% boric acid due to the MgAl and Sodium Nitrate. Not sure what is happening here but obviously a reaction of some kind. Other chems are just Sulphur and Charcoal. Water and dextrin to bind.

 

 

 

 

 

[Rob.L.]

Just out of interest is it definitely expanding or is the drying layer contracting?

 

If expanding It could suggest a gaseous reaction of some kind? Have you or can you test for the presence of any evolved gas? Is another solvent miscible with water suitable for this comp without an adverse effect?

 

I don't have experience of this, just some thoughts based on simple chemistry.

[cooperman435]

With my experience it was definitely expanding metal issues. There was a clear growing white oxide fuzz around each grain

[Tinderbox]

Deffo an expansion. The only solvent was water. Hasn't happened to the KnO3 version of the same formula. Phil suggests coating the MgAl. I just need to know how that's done and I'll give it a whirl.

[Pyro-Gear]

Your problems are very common with this sodium nitrate based stars, what you have is a driven in star and the reaction between the alloys is pushing out cuseing splits in the stars, sodium nitrate is very hydroscopic so dry it first!

However if you knock out the Dextrin and sub that for PVB and bind with IPA you won’t get the problem also your stars won’t burn to the ground regards Ken. 

Edited by Pyro-Gear, 01 February 2018 - 06:11 PM.

[Tinderbox]

Cheers Ken. I am all too aware of the hygroscopicity of NaO3. My innitial tests were with red gum and IPA but the stars just didn't have any structural integrity and burned slowly. I would love to try PVB. Any suggestions of sources on that? Yourself?

[Pyro-Gear]

Hi Red gum is not a very good binder, most formulations use it more as a fuel donator.

I have been working with PVB for at least 3 years now and have developed a lot of formulations colour and strobe that can be found in AFN, most of the formulas use 2% PVB and that is more than enough to set the stars rock hard in 24hrs.

Yes I have PVB in stock so feel free to contact me should you need some regards Ken.   

[Tinderbox]

Despite the apparent reaction between the MgAl and NaO3 resulting in the cracked and expanded stars, they perform really well. Sent a hand-full up in a soft break bouquet/horse tail style skyrocket last night. very beautiful effect. Not sure how they'd perform under hard breaking shell conditions. I feel they may well get smashed into dust. A very nice effect all the same. Looks like a very fine deep vibrant yellow glitter dust with a substantial hang-time.

 

Ken. I will PM you.

[Tinderbox]

Cheers for the PVB Ken. Arrived today. Looking forwards to using it.