"You must learn from the mistakes of others. You will never have time to make them all yourself."
I would add that some mistakes can cost us the opportunity to make others. Tacky little things like death, disability, imprisonment, any of which could result from poorly performed rocketry experiments.
Safety and freedom are opposites. More of one is almost always less of the other. The challenge is to find a dynamic balance: Enough safety to keep us alive, whole, and out of jail but allowing enough freedom to be, to learn, and and to grow that life remains worth living.
In order to enjoy the freedom to do experimental rocketry, we must devote substantial effort to safety precautions. Otherwise, the freedom will be taken from us, either by our community or by the events themselves. This applies to individuals who perform irresponsible experiments and suffer disability or legal injunction, both limiting to subsequent freedom. Or it can apply to experimentalists as a group through restrictive laws and policies imposed in response to unfortunate incidents.
We enter a strange and dangerous world, and must seek the guidance of those who have come before.
Here is the little bit that I know.
Basic Safety Precautions
1. Not in kitchen. I expect this will create a dilemma for many prospective propellant makers since the process requires the use of an oven, and most ovens are found in kitchens. I hope one day to demonstrate that this process is safe enough that small batches can be made in an average kitchen, but that will require extensive testing and documentation which I have not yet done. So for now, I recommend adding an oven to your workshop or lab.
Accidents may not be more likely in a properly-prepared kitchen than a lab, but the consequences are likely to be much greater. And then there is the issue of the "owner" of the kitchen, which isn't necessarily the person who makes the mortgage payments.
2. Small batches.
This offers both a lower chance of accidental ignition (less surface area to catch a spark) and less flash and smoke if the batch does catch fire. Making larger batches requires more extensive safety precautions than described here.
3. Protective gear
At certain points in the process, you will be handling the propellant. At these points, you must wear the face shield and other protective equipment. If the fuel somehow catches fire, it can burn very quickly. Any person who is nearby is likely to be burned by the flash. Protective gear can mean the difference between a surprise and a tragedy.
4 and 5. Screen your company
Most children and some pets are very distracting. They can cause things to move about unexpectedly, and can be burned or otherwise injured in an accident. If there are kinetic children in the vicinity, make sure a competent and diligent person is watching them in another room with the door closed. If this situation cannot be arranged, it is a bad time to make rocket propellant.
Other adults may want to observe. That may be OK, provided that they are observers/helpers and not distractors/spark sources. Any adult present must know what is going on, know the risks involved, and know the procedures to follow if an accidental ignition occurs. They must agree to avoid distracting you from the propellant-making process, and must leave their cigarettes outside the work area.
6 and 7. Be vigilant about ignition
I often envision a little elf who is a heavy smoker and jumps about at will, dropping sparks everywhere. This is to maintain the mindset that assumes my propellant can ignite at any time, for no apparent reason. I will take care to ensure that if there is an accidental ignition at any point in the process, no one will be hurt. "I can't imagine how it could go off!" reflects a lack of imagination. It can go off, and it doesn't need a reason.
All that said, recrystallized KN/sucrose seems to be one of the least likely candidates for spontaneous ignition, and that is one of the reasons I still make it. But the "it-can-ignite-anytime-for-no-reason" mindset is a great safety feature, and I recommend you adopt it.
8. Clear your workspace
Flammable things nearby offer substantial opportunity for a major fire. This rocket propellant burns fast - its flame will be gone in a few seconds. It is not likely to ignite heavy wood or plastic objects, but if there are cup-towels, paper towels, newspaper, or other easily ignitable items nearby, the burning fuel may set fire to them. If you do this in a workshop, make sure any volatile liquids are removed before beginning the process. Don't forget the lawn-mower gasoline!
9. Fire extinguishers
Having a fire extinguisher around is a good idea in general, and a really good idea if you are making rocket propellant. I recommend at least a 5-pound dry chemical extinguisher. This is not so much for putting out the burning propellant - most likely it will be all burned up before you can react. The extinguisher would be used to put out the things the propellant sets on fire.
The bucket of water is good for dropping hot things into in an emergency, and can serve as a secondary extinguisher in a pinch. If you are working on a batch of propellant and it ignites, is usually best to step back and wait for it to burn out, then engage in damage control. If you happen to be holding the propellent when it ignites, drop it immediately and step back. The bucket of water gives you a good place to drop it.
I also keep a garden hose outside the workshop door.
10. Ventilation and escape
This propellant produces a dense, lingering smoke. A small batch burning in a closed area will create a cloud that obscures vision completely. It is also irritating, and can be toxic at high concentrations. So it is important to know your area well enough to be able to escape to open air without use of vision, to be able to ventilate the room and let the smoke out, and to be able to extinguish any secondary fires without exposing yourself to great personal risk.
11. Avoid Stupid Tricks. Unfortunately, the difference between a normal procedure and a stupid trick may not be obvious at first. More unfortunately, this kind of experience may come at great cost.
I cannot describe all possible stupid tricks. I will point out those of which I am aware, but it is up to you to think ahead about any procedure you are considering, and wonder: What could go wrong? What is the danger? How can I protect myself, others, and property in case this thing goes wrong?
Here are a few "stupid trick" warnings for starters:
12. Avoid Excessive Exposure to KNO3
Eating it: Even though it is considered "non-toxic" by the US FDA, ingesting quantities of potassium nitrate may not be a good idea. There is evidence that the use of nitrates/nitrites in foods can cause cancer. In fact, some countries have banned their use in food, and others are considering such a ban. It has been found that heating protein in the presence of nitrate or nitrite (e.g. frying bacon) creates nitrosamines, which are carcinogenic. Demographic studies indicate a dramatic increase in colorectal cancer occurred in the mid-1800s when the use of nitrates/nitrites became widespread in meat processing. I understand that tobacco companies added KNO3 to cigarette tobacco to make them burn more steadily, perhaps contributing to their carcinogenicity.
There is also a concern that bacteria in the stomach can convert potassium nitrate into the more toxic potassium nitrite. Nitrite oxidizes the iron in hemoglobin to create methemoglobin which cannot carry oxygen as normal hemoglobin does. Some methemoglobin is normal, but excessive conversion of hemoglobin causes methemoglobinemia. Many other chemicals can cause this disorder, including other nitrates, chlorates, and perchlorates. Methemoglobinemia is considered especially dangerous in infants, where is is called "blue-baby syndrome" because of its most apparent symptom, a bluish hue to the lips, hands and feet. Infants under six months are are more susceptible because the higher pH of an infant's stomach is more friendly to bacteria, thus nitrate is more readily converted into nitrite. It is thought that nitrates from polluted groundwater may induce this disorder in infants.
So I discourage intentional consumption of KNO3. Please keep it
away from children and don't feed it to the baby!
Also consider if you are using a technical grade or fertilizer grade it might contain other, less benign chemicals.
Touching it: Apparently, some people are very sensitive to potassium nitrate, and can develop a form of dermatitis upon contact. Most people don't, and can handle it without problems. But if you find yourself itching and scratching after using it, consider the possibility that you are one of the sensitive few and try using gloves.
Breathing it: Inhaling large amounts of KNO3 dust has been traced to some lung disorders. If you are handling large amounts in a closed environment or are exposed to KNO3 over a long period of time, using a dust respirator would certainly be in order.
Smoke: Burning KN/sucrose yields large volumes of dense white smoke. The white stuff is mostly fine particles of potassium carbonate but also contains some potassium hydroxide. The carbonate of potassium is mildly caustic, the hydroxide is very caustic. Thus breathing large amounts of the smoke could be problematic.
Here are links to a couple of MSDS forms for Potassium Nitrate:
For comparison, here is JT Baker's MSDS for table salt
Note that KNO3 and NaCl are given the same hazard rating for contact and health in general
Some sanity: The good news is that most people are not particularly sensitive to KNO3 and can handle it without problem. Since it is mildly hygroscopic, it is not a particularly dusty substance unless intentionally dried. It is very water-soluble, and so easy to clean up. And since this is a recipe for rocket propellant, the carcinogen factor should not be a big concern as long as you keep it out of your food.
Mixing dry KN/sucrose is relatively safe in and of itself. The mixture
is slow-buring, not sensitive, and not known to auto-ignite.
But in a closed space, large amounts burning could present a danger through flame or smoke, or by setting other things afire.
The components of this propellant are inherently non-toxic. Potassium
Nitrate is used as a medicine, a meat preservative, and in desensitizing
toothpaste. Small amounts can be injested without immediate harm
(but see cautionary note above.) Sugar is another matter. Many
nutritionists consider it poisonous, although I have known some people
to consume substantial quantities and survive. At least for a little
Danger Point 1: Dissolving the solids
Risk level: low to moderate
The process involves mixing KN/sucrose/corn syrup with a little water and dissolving it over heat.
I believe this to be relatively safe, having experienced no unfortunate incidents in hundreds of batches, BUT!
Don't leave the pan unattended while heating! If all the water
cooks out, it could ignite.
Don't heat it too vigorously! If it boils over, liquid reaching the burner could dry out enough to ignite. It is not necessary to boil the mix, just heat it enough to dissolve the KN and sucrose.
Danger Point 2: Baking pans to dryness
As the pans bake, the mix becomes dryer. At some point it becomes capable of sustaining combustion.
Risk A: If you are baking propellant and another matter demands your attention, turn the oven off. You can always resume baking later. But if the mix is nearing dryness, you must be very protective. Remove the pans from the oven, cover with plates and place in a safe spot.
Risk B: There is some danger when taking samples from the pans to test for dryness. Even more risky is tossing the sample back into the pan. Once I missed the pan. The sample landed on the oven floor. I shut the door, turned off the oven, backed away and grabbed the fire extinguisher. The sample turned into a black blot but did not ignite. Whew.
Danger point 3: Scraping it up
Risk level: High!
I believe that scraping dried KN/sucrose out of the pans is the most dangerous step in the recrystallization process.
This is because:
Danger Point 4: Re-heating
Recrystallized KN/sucrose can be re-heated and re-formed almost indefinately
Stupidity alert! Don't re-heat in the microwave! The propellant will ignite. Toaster-oven is recommended.
Danger: Contact with heating element.
Recrystallized KN/sucrose seems to be very stable and insensitive to shock or friction. It must be sealed air-tight to protect from moisture - this seal should also protect it from flame or stray sparks.
Danger: Recrystallized KN/sucrose can be ignited by external sources or spark, flame, or heat. The resulting fire will be brief but intense, and will likely set other things on fire, such as its plastic container.
This is an incomplete list. There are more dangers than I can imagine, much less describe. I leave it to you to learn the nature of this propellant, its virtues, and its risks to your physical and social environments.
Respect will come from knowing it well. Knowing it will come from making many small batches and experimenting with them cautiously.
Please be safe.