Of all of the available effects to the pyrotechnist, flash powder is probably the most fascinating and certainly the deadliest effect we have. Perhaps this is due to the awesome energy potentials, the blinding flashes and the heart stopping blasts furnished by it. I bet that more people in the pyro trade have made flash before they have made stars or aerial shells. Fireworks makers in the US ,themselves, use more flash as a component and as an effect than any other star or effect.
"Accidents" involving flash in both the legal and illegal trade are often severe and catastrophic. The events leading to a flash disaster are all to common: complacency, laziness, forgetfulness, carelessness, etc. These, accompanied by blind trust ("I've never had a problem before") are a recipe for disaster. A truck plowing into your magazine is an accident, the rest is negligence.
A decent flash has a critical detonable mass of between 30 and 50 grams. This means that that quantity will detonate with concussion and a shock wave when ignited in open air: no container, just loose on the ground. A cup or a shell casing does not count as loose: it still confines the flash enough to accelerate the reaction. Less flash than the critical mass will just burn violently. Compare this with black powder which has a critical mass of over 500 pounds. The average three inch salute containing about 4 ounces of flash will dismember a person, not just blow off a hand. Think about it the next time you blend an 11 pound batch! One of the really nasty realities about flash is that as the size of the charge doubles, the force of the explosion increases eight times! Those of you who have seen the flash demos at PGI conventions since '85 or so will remember what a pound of loose flash does to a wooden structure. The flash, fireball, and chest pounding report is an awesome sight. You generally can't find a piece of the shed bigger than a baseball.

Here are a few of the known do's and don'ts regarding flash:

Do mix only in humid weather (relative humidity of 50% or greater) to reduce the hazards associated with static electricity ignition.

Do wear only cotton or leather clothing when mixing (burn and static potentials).

Do remove all jewelry and metal from your person.

Do spray yourself, your work area, and your tools with static guard laundry spray before mixing.

Do screen all of your ingredients separately. Never screen compounded flash (or other high energy materials). Particularly, you should also never screen anything with a sparking material such as titanium in it.

Do mix flash on a large sheet of paper by alternately lifting the corners and rolling the ingredients to the center. This method, known as the 'diaper' or 'blanket-rolling' method is common throughout the fireworks and the explosives trades and is always used for high energy and sensitive compounds.

Do add the titanium last, after the material is well blended (I also add some rice hulls to flash for 3" and larger salutes to prevent caking).

Do mix outdoors, away from people, buildings, etc.

Do limit your batch sizes to 10 pounds or less (commercial guidelines).

Do limit one batch at a time and one worker in a work room when charging casings.

Do remove all charged casing to a magazine before introducing a new batch to the work room.

Do clean up all spills promptly and be very careful cleaning spills containing titanium.

Do wear a dust respirator when mixing: metal dusts have proven to be poisonous.

Don't mix or store flash in anything plastic, or use any plastic tools or utensils (static hazard).

Don't store loose flash, particularly in bulk.

Don't mix, store, handle, or use any flash containing potassium chlorate or magnesium in it, particularly if sulfur is present in the mix. Note that paper usually has a good bit of free sulfur present in it

Don't mix where flash can be blown away to be ignited by a pilot light, etc.

Don't smoke or be around any other source of ignition if you are wearing clothes that have been possibly contaminated by flash.

Don't expose any extraneous people to flash operations. Limit it to just the people needed to get the job done, period.

From a safety stand point, enough just can't be said about flash powder. If you don't believe me, just check out the John Vico seminar in the AFN Fireworks Video Magazine #4. He is living proof that shit can happen to anybody. You just can't have the "ain't gonna happen to me" attitude. At one of our club picnics I set off a bag of flash powder (a little less than one pound). You'd have to be there to understand the power of this stuff. This powder was in a bag - no casing, no end plugs. For this small demo you will gain an instant respect for flash. To wrap up this short article, I'd like to add some info that was not included in Mike Stoumen's Flash article. The following is reprinted with permission from Bill Ofca's "Technique in Fire: All about Flash and Aerial Salutes." This is a breakdown of what will happen when one hundred pounds (100 lbs) of flash is detonated in the open air. It is instantly lethal in a 25 foot open air radius and lethal up to several hundred feet if hit by missiles propelled from the blast. Windows will break for a 1/4 mile radius and buildings will sustain structural damage to window and door frames up to 600 feet away. Buildings within 200 feet will sustain structural damage to framing timbers. Any questions?????