8 replies to this topic

[overflow]

Hello to all.

I tried searching the forums but searching for mixing in the titles gives only a couple of threads (and searching it in the whole post is useless since it's quite a common word in pyrotechics) and "mix" is too short.

What i wanted to know is what are the techniques to mix chemicals and for how much time should the mixing take place.
Reading around i found the following:
-mortar&pestle (mainly in old books, can take quite a long time and many mixtures can't be mixed this way due to the risk of detonating on impact with the pestle)
-Diapering (for sensitive mixtures, for how much?)
-Screening (for less sensitive mixtures, for how much?)
-Ball milling (speaking of mixing it is only used for black powder, 2-3 hours to effectively mix)

Am i missing other methods?
For how much time should i continue to mix with the diapering and screening techiniques?


Today i was trying to mix some coloured fire (i already did bp with the CIA method quite a few times)
I found this formula with easy to obtain chemicals ( Wouter's Formula Page):

Yellow fire composition #1
Source: rec.pyrotechnics. Composition from "Magic With Chemistry"[7], chapter "colored fires"
Comments:
Preparation:

Potassium nitrate.................................4
Sulfur............................................1
Charcoal..........................................2
Sodium chloride...................................3

So i mixed a small quantity of it (10grams) and went mortar&pestle on it for a couple minutes, after a bit i noticed that it was too little to mix well with the mortar&pestle and so tried the screening method for a couple times.
I then tried out a bit of the powder just to find out that it bubbled and burned with hickups (much like green powder).

My guess is that i needs more mixing, am i wrong?

Thanks!

Edited by overflow, 19 January 2007 - 01:17 PM.

[BrightStar]

Hi overflow, welcome to the forum.

The various methods of grinding, milling, mixing and binding are integral to the art of pyrotechnics. Ultimately, you learn with practice and time.

Given your example yellow formula, I would probably mix it as follows:

- Weigh out the KNO3 and NaCl together. Dry them if necessary in a warm oven. They are likely to be fairly coarse, crystalline solids, so place them together in a pestle and mortar and grind until as fine as white flour. The reactivity of oxidisers is very much dependent on the particle size and generally, if they look like table salt they are too coarse and need grinding.

- Separately weigh out the charcoal and sulfur. If they are already fine powders, set aside, if not grind in the pestle & mortar, having tipped out the oxidisers (never grind together...).

- Now mix the two parts thoroughly with a plastic spoon and sift them through a 20-mesh screen to break up any lumps. Stir the mixture and then sift it through a 40-mesh screen. Stir again. Some would suggest passing it through an 80-mesh screen but it's usually not needed.

- I would test the comp by hand pressing it into a thin walled tube, priming with BP and fusing with visco.

This illustrates the basics... Use your pestle and mortar to grind chemicals that appear coarse until they are as fine as flour, but don't grind fuels and oxidisers together. Once you have your chems fine enough, use a series of screens to remove any lumps and mix them thoroughly.

Diapering is used for very sensitive compositions - particularly flash, where the friction of screening might be hazardous. The ball mill is used with individual chemicals to reduce the particle size - much like the pestle and mortar. You can use it for example to reduce a batch of coarse KNO3 for use in fountains etc with no further grinding. With certain chemical combinations - charcoal stars, BP etc, it can be used to grind, mix and combine the chemicals simultaneously, but for most compositions this is too risky.

Hope this helps - now search and read more!

Edited by BrightStar, 20 January 2007 - 06:03 AM.

[overflow]

Thanks for the informative reply!

You guessed well, both KNO3 and NaCl where grainy, i will try to powder them with the mortar&pestle... speaking of which mine is in wood and i guess it's not the best for the job; i was thinking of getting a marble one, what do you think? Marble shouldn't make sparks (at least if the pestel is in wood).

Actually i'm having a hard time pulverizing the chemicals in the wooden mortar (i think the wood is too soft), is it acceptable to put the KNO3 and NaCl (not togheter) in a kitchen blender?

This illustrates the basics... Use your pestle and mortar to grind chemicals that appear coarse until they are as fine as flour, but don't grind fuels and oxidisers together.

Are there exceptions to the "not grind fuel&oxidizer together" ? I'm asking because i thought the bp like compositions were quite safe

[UPDATE]
So i actually powdered into flour the KNO3 and NaCl (separately) and then mixed them with the other chemicals (already well powdered).
I then inserted the powder in tube of little diameter, primed it and then set to fire with a match.
After an initial yellow flame the effect stopped and the rest of the tube just went on fire.

I then tried to fire 10g unconfined of it, it burned not so fast but steadily.
The real weird thing is that at the end i was left with a lot of residue that rapidly solified into something that looked like lava rock, my guess is that this residue impeded the powder to burn properly in the small tube.

Is this residue a common mistake of something due to the formula?

Edited by overflow, 20 January 2007 - 05:30 PM.

[portfire]

Hi overflow and welcome to the forum. Although bp is a relatively safe composition to work with it should never be mixed by hand, just in case of accidental ignition. Ball milling bp is the only way to get a good
quality meal (apart from the CIA method which both milling and the CIA method are covered in other topics)
and should ONLY be done remotely.


As for the residue (potassium nitride i think), this is because the fuel/oxidizer are not mixed intimately, i suppose you could say it's not the complete reaction you need and ball milling or cia will give you the reactive bp you want.

Hope i'v been of some help

ps some nice info there BrightStar

Edited by portfire, 21 January 2007 - 10:55 AM.

[Mumbles]

The pearls you speak of from BP are more than likely not potassium nitride. Nitrite maybe,

As far as a kitchen blender, yes it is safe to put KNO3 in one. I would suggest getting your own pyro devoted blender though. The signifigant other or parent may not enjoy their blender being used for pyro purposes. You could safely grind the KNO3 and NaCl together in the blender. The NaCl offers no fuel value.

I'm not sure what is going wrong with your devices though. It sounds to me like you're using too thick of a tube, and there is slag buildup. It might be more of a lance composition. They need very thin wall tubes. 2-3 turns of computer paper or notebook paper would be acceptable. Fold or crimp the end and just press in with a pencil or something.

[overflow]

I was actually referring to the yellow flame composition, not bp

This is the residue i get (sorry for the low quality):

(in weight it is like 50%-60% of the original mixture)
Any ideas?

As far as a kitchen blender, yes it is safe to put KNO3 in one. I would suggest getting your own pyro devoted blender though. The signifigant other or parent may not enjoy their blender being used for pyro purposes. You could safely grind the KNO3 and NaCl together in the blender. The NaCl offers no fuel value.

Ah, yes, good advice. I used the kitchen one because i know that KNO3 can be easily cleaned and is not poisonous (at least not in small quantities... but then again even too much water can kill you).
Good to know i can mix the two salts together without risks.

Edited by overflow, 22 January 2007 - 12:26 PM.

[Mumbles]

I wouldn't grind more together than you plan you use at once. There may be some NaNO3 formed over time, which may cause the comp to fail.

[FrankRizzo]

From your description and looking at the components of that formula, the slag is most likely molten salt. 3 parts salts seems quite excessive; try using 1 part instead.

[overflow]

Thanks, i'll try changing the formula and i'll let you know.