Best way to roll stars containing Mg and Al?

[FirecrackerXL]

Hello,

So I've been wanting to make red and white stars for a while now, but this is holding me back. The composition I want to use for the red stars

contain magnesium, strontium carbonate and a tiny bit of potassium nitrate, in the white stars I use aluminium (a little bit of flake, the rest spherical). My question is how do

you roll these stars safely? Because magnesium/ aluminium and water can create self igniting gasses. I think coating the MG would make the stars more prone to breaking.

Is it better to use a alcohol soluble binder, like shellac or phenolic resin?

This of course won't really be cost effective since you would have to use 99% alcohol, and that stuff evaporates quickly.

Thanks in advance!

Greetings

[Carbon796]

The Mag should be coated for water binding. You could try swapping to Mg/Al, to avoid that issue.

 

The aluminum doesn't need any protection for water binding. In the majority of comps.

[Mumbles]

I'd agree with Carbon. It may also help to keep things even in terms of brightness and burn time to try to get formulas that are similar or created off of the same base. Doing something like that will keep any sort of treatment requirements the same.

[FirecrackerXL]

Thank you so much guys, MgAl is a really good one. Im not sure if it will produce the same colours, but I'll look into it. I will also look into similar compositions.

 

Thats really interesting, I never knew you wouldn't have to treat Al stars. When I do make stars containing KNO3 and Al I do always add 2-3% boric acid, better safe then sorry, haha!

 

Thanks!

[Carbon796]

Boric acid is really only needed. In a potassium nitrate / aluminum comp. When the comp is too basic/alkaline. Especially with fine reactive flake. Otherwise, the majority of the time its not needed. Do not use boric acid with magnesium or Mg/Al. As it's more likely to cause a reaction, and offers no protective qualities for either one. Edited September 21, 2020 by Carbon796

[SharkWhisperer]

Several folks here have described exothermic (heating) reactions occurring with nitrate/Al mixtures, particularly when comps are wetted. You can find threads here on the topic--just search "boric acid". Many others haven't experienced this and avoid the extra addition of boric acid. I don't feel strongly either way: I have boric acid and have used it (when I thought it was a mandatory safety precaution--a long time ago but not since).I still have much of my first 1-pound bag remaining; haven't used it in years. It's cheap insurance but an extra step, and possibly a waste of time. Especially if you're using bright (i.e., coated, often with stearin/stearic acid) Al that already provides an added measure of protection. Regardless, 2-3% boric acid will almost certainly have an inhibitory effect on your comp's burn rate, not to mention potentially affecting color saturation, etc. That is A LOT of boric acid--probably unnecessarily so. If I were to use boric acid these days in similar comps, I'd mix it in to 0.5% final percentage. If using water for wetting, I'd just dissolve the boric acid in the water wetting solution--no extra chem to sieve/mill, and get much more effective distribution throughout your powder. Expect a noticeable difference in performance characteristics/burn rate if dropping boric acid from 3% to 1% or less. I've stored nitrate/Al comps and constructions for years without incident. You choose what you're comfortable with, hopefully after reading through real experiences by fellow pyros. A few might swear by it. Most don't. Still, it does qualify as "cheap insurance"...

Edited September 21, 2020 by SharkWhisperer

[FirecrackerXL]

Yes, I only use boric acid for a comp containing potassium nitrate and Al, never for anything else. But I didn't know it could actually cause a reaction (in comp's containing Mg and MgAl), interesting. I generally use a bit of 9 µm german dark in my composition to get the stars burning. I also never knew that MgAl is pretty stable, I've always avoided comp's containing it, just like magnesium. Good to know.

 

I can imagine the burn rate and color saturation being affected because of adding too much boric acid, thank you for the advice. I don't use coated aluminium. I use 4 µm chinese flake or 9 µm german dark, generally 9µm because it burns a lot slower. The spherical Al I use isn't reactive at all. I always like to be on the safe side, so I'll stick to 0.7% - 1.2%, but I'll do some tests on this. Adding the boric acid to the water and alcohol solution is a really good idea.

Edited September 21, 2020 by FirecrackerXL

[SharkWhisperer]

How large is your spherical Al? I ask because the 5-um stuff I get from Alpha Chemicals makes a very reactive FP, not quite 2-3um Indian/German dark speed/brisance but surprisingly close behind.

 

Perhaps with boric acid, make it easiest to calculate and just add it at 1% to keep the math simple. Almost certainly you'll notice a diff between 1% and 3%, but much less likely you'd notice a difference between 0,7% and 1%. For sound protection, with good distribution (particularly if in a solvent), I would expect that 1% BA would be more than sufficient. I have NEVER heard of a nitrate/Al comp overheating or reacting when it contained 1%, or even 0.5%, BA. If using in powder format, you'll want the finest powder you can achieve--a coffee grinder is probably sufficient. Don't breath in the airborne dust when you open it after grinding--let it sit a minute or two to settle first.

 

Slow flash recipes using KNO₃/Al/S are used for various purposes in Chinese commercial stuff, and by many of us amateurs, too. I've never added boric acid, have kept small quantities sealed up for months with no issue, is rarely mentioned in recipes (although there are several threads here on APC on the topic; one is: https://www.amateurpyro.com/forums/topic/13245-kno3als-flash-powder-storage-stability/?do=findComment&comment=182274

 

I was unaware of reactivity between boric acid and MgAl like Carbon mentioned above--does anybody have additional info on possible reactions between these two chems and their mechanisms?

[FirecrackerXL]

That is a really good question. I bought it a long time ago, but I don't think it was pyrotechnical grade. I do know it is 99,9% pure. Looking at the powder it self and the tests I did I confirmed it was spherical. It is basically impossible to make flash with it, it just doesn't light, even with a blowtorch. Lets say if you threw it in the air it wouldn't give off a cloud, and it doesn't compress. Thats all I know about it.

 

1% sounds good, yes. I can imagine that would also suffice. I usually dry my boric acid (in dry comp's) and use a mortar and pestle to make it fine, there isn't much to grind anyway.

 

That topic is really good, I've read that multiple times in the past. I also use KNO₃/Al/S flash, for that I usually put in 2.5% boric acid. I try to do 2% but I always ''accidentally'' spill some more in, haha!

In my FP I don't notice a difference in the burn rate at all, even between 0% and 3% boric acid. But I can imagine there would be a huge difference in stars.

Edited September 22, 2020 by FirecrackerXL

[kingkama]

Usually spherical al is 325 mesh or 45 micron, good for tail and glitter.

[SharkWhisperer]

My only sperical is Alpha's 5 um. They also sell a 30 um that supposedly works ok in reactive targets (most use the smaller size, though), and probably for effects in some star comps, but I've never tried it and would expect it to be way too big for flash. Kama's 45 um spherical would be even worse for flash purposes, but is very useful for star effects.

 

The 5 um spherical I use is super shiny (i.e., likely coated) goes airborne just unzipping the bag, and produces a surprising flash effect with perc. But I also have tons of German/Indian/Chinese flake that's made for that purpose and performs as expected. The Indian from DeepDixie (Cock brand; I think from India) is probably as fast as I've ever seen--slightly edges out the German Ekart flake I have that costs more. All of my FP small flake pyro aluminums are are super fine, too, but don't really seem to go airfloat. Might be that the coating on bright allows the particles to separate from one another more easily. Might be that it's just harder to see because it's not shiny. Who knows?

 

To some blackmatch batches, I've added 5-10% Al to the BP comp to heat up the burn just a bit--which gets very wet obviously before coating string--and sun dried on hot days without boric acid and without any issue. Just hot/sparky blackmatch. Stored some for years in cardboard tube with a few drierite packets, and works great even after long storage. Never had a spontaneous reaction. Just another example of my experiences not using boric acid in nitrate/BP/Al mixes. But boric acid is cheap and easy to handle. If mixing/screening in dry, I'd probably want it smaller that I could get using a mortar and pestle. The finer the particle, the more Al surface area a given amount can contact and protect. Hence the coffee grinder (or a mill if you're really anal about size...). Screening in will always give better mixtures faster than other hand mixing methods.

 

Never even considered pre-drying boric acid. At 1%, can't imagine any water it could attract would set the overall comp weight/% off by much, or affect burn, but to each their own. If it does contain water, I could see it being more prone to clumping/resistant to fine milling, though. I religiously oven dry out my KNO₃, strontium nitrate, and charcoal before mixing red comps or making BP and BP-based stars. Boric acid, too? Ok, no harm done. I just wouldn't bother personally, but many also don't dry the chems that I do. Whatever works for you.

 

Also, still curious if anybody has any information on whether boric acid might react with MgAl, and if so, how?

Edited September 22, 2020 by SharkWhisperer

[FirecrackerXL]

Yes there is a big chance my Al is in the 45 micron range.

 

I think that is because it is darker, like you said its harder to see. It does probably go airfloat.

My mortar and pestle get the powder really fine, maybe even in the single digit micrometer range. Making it as fine as possible is important, yes. If I make a specific batch containing KNO3 and Al I just add the boric acid to the KNO3 when milling, that makes it even easier. When I don't, I screen it into the batch, not with FP of course, haha! I can understand not using boric acid, but it still takes no time to add it, and by doing so you could potentially save your house from burning down, that alone is worth it 100% for me.

 

Drying the boric acid does in fact make the milling a lot easier. It also spreads way easier through the comp, and it does change the weight a little bit. I also dry my kno3, I don't really dry my charcoal, because I vacuum seal it after I'm done making it.

Edited September 23, 2020 by FirecrackerXL

[Mumbles]

I typically just add boric acid to the wetting water. You can only get maybe 3-4 wt% to dissolve normally, even though it has a higher reported solubility in some places. It's much more effective at buffering when it's in solution. The Nitrate/Al reaction is mediated by water, so it wont really happen once a composition is dried anyway. I've only had an issue with a few select compositions, and they all contain additional basic components like sodium oxalate or sodium bicarbonate. They usually were based on flake Aluminum as well. Boric acid really doesn't affect the burn rate noticeably, and certainly not the color.

 

Boric acid does react with MgAl. This, like all reactions, is even worse if heated. Boric acid works by soaking up hydroxide to make borates. This is how it mediates the nitrate/Al reaction. The same property can cause it to react with MgAl. Aluminum oxide itself is somewhat amphoteric due to how much it likes to hold onto it's oxygen. It is decently acidic which allows hydroxide to easily attack and makes aluminates, which expose fresh Aluminum and can continue to react. However, this oxide layer tends to be fairly inert to weak acids since it does not necessarily want to give up it's oxygen. You need something fairly strong to dissolve it. Magnesium is not the same. It is also amphoteric to some degree, but it's oxide layer is both flakier and more basic. This makes it more prone to reacting with acids or water. It is this property that makes MgAl more prone to attack or will allow boric acid to dissolve it's oxide layer. MgAl is more prone to react than aluminum, but less so than pure magnesium.

[SharkWhisperer]

I typically just add boric acid to the wetting water. You can only get maybe 3-4 wt% to dissolve normally, even though it has a higher reported solubility in some places. It's much more effective at buffering when it's in solution. The Nitrate/Al reaction is mediated by water, so it wont really happen once a composition is dried anyway. I've only had an issue with a few select compositions, and they all contain additional basic components like sodium oxalate or sodium bicarbonate. They usually were based on flake Aluminum as well. Boric acid really doesn't affect the burn rate noticeably, and certainly not the color.

 

Boric acid does react with MgAl. This, like all reactions, is even worse if heated. Boric acid works by soaking up hydroxide to make borates. This is how it mediates the nitrate/Al reaction. The same property can cause it to react with MgAl. Aluminum oxide itself is somewhat amphoteric due to how much it likes to hold onto it's oxygen. It is decently acidic which allows hydroxide to easily attack and makes aluminates, which expose fresh Aluminum and can continue to react. However, this oxide layer tends to be fairly inert to weak acids since it does not necessarily want to give up it's oxygen. You need something fairly strong to dissolve it. Magnesium is not the same. It is also amphoteric to some degree, but it's oxide layer is both flakier and more basic. This makes it more prone to reacting with acids or water. It is this property that makes MgAl more prone to attack or will allow boric acid to dissolve it's oxide layer. MgAl is more prone to react than aluminum, but less so than pure magnesium.

Appreciate the clear answer re boric acid/MgAl reactivity. Makes good sense. Tx.