An interesting observation

[Richtee]

So, after months of drying in a low humidity area, I was looking at some BP pucks I had made. They seemed to lighten quite a bit in color. So, I took a utility knife blade and shaved them. The resultant dust was very light grey and burned slower than the mill dust would have and was pretty drossy.

Hmmm... now I typically scrape green pucks because there’s always some “flash” from the pucking/pressing process at the edges. This would become dust/weak powder upon corning anyway. I would not have expected much migration of the nitrate after that, but...apparently..it’s more than I expected.

 

Any O you more chemiclly inclined than I have any insight?

[MadMat]

Let me guess, you didn't add any alcohol to the water you used to wet your milldust before pressing your pucks, and/or you used a little to much water to wet the milldust before pressing. It sounds like you dissolved too much KNO3 when you wet the mill dust. Adding alcohol to the water, lowers the solubility of KNO3 a considerable amount. Adding too much water will also cause problems with dissolving the KNO3. The biggest problem is that dissolved ionic salts such as KNO3 will recrystallize as the water evaporates. The slower the water evaporates, the larger the crystals will grow. When this happens you pretty much negate all the ball milling that was used to grind the KNO3 particles to the smallest size possible for more intimate contact with the charcoal and sulfur. That's why the surface looked lighter in color and left a lot of dross when burned. That's my take on it anyway.

I always add about 10% alcohol to the water and add only enough to my milldust to stop if from being "dusty". I add such little water, that I never have any liquid come out of the pucks while they are being pressed. If you do this you will find that your pucks dry out much faster. Additionally, I break up my pucks after only a short time of drying. This does two things, it makes it easier to break them AND any remaining moisture dries rapidly, avoiding the large crystallization caused by long drawn out drying.

Edited May 5 by MadMat

[Richtee]

I use 50/50 water methanol, and only add enough to ‘de-dust”. Which is why I’m kinda perplexed. I did press a dry puck. While it appeared OK.. corning it was a disaster.

 

Perhaps I need to “force dry”. I have a spare tray for my jerky drier...

Edited May 6 by Richtee

[MadMat]

Apparently you are doing everything right. Sorry if I seemed condescending in any way. I didn't mean it! Maybe the timing on breaking up and corning those pucks is important. I always chose a dry (low relative humidity), sunny day to press and corn my milldust. Once pressed I set the pucks out in the sun with a small fan on them for about an hour or two and then break them up. If I don't get the weather conditions I prefer, I do have an food dehydrator with screening on the shelves I can use in a pinch. But once corned and graded, I put the bp into glass jars, cover them and put them in the sun for around 15 minutes. If I see any dew on the inside of the jar or smell any alcohol at all, the bp goes on a screen and sits out in the sun (with a small fan) for a while longer. I truly think that corning before being completely dry helps out a lot. This makes the water dry out very fast and stops any of that nasty recrystallization

 

 

 

Edited May 6 by MadMat

[Richtee]

7 hours ago, MadMat said:

I truly think that corning before being completely dry helps out a lot. This makes the water dry out very fast and stops any of that nasty recrystallization

 

 

 

You may be onto something there. I tend to corn “as needed” pretty much. Perhaps letting pucks sit around is not the way to go. Huh... well, guess I’ll go get dirty later

 

And I took no offense. Matter of fact- thank you :)

[DavidF]

My guess is that it's a density issue. Pucks pressed to 1.7+g/cc don't have that issue. If a puck is weighed, dipped in 99% isopropyl alcohol, wiped dry and re-weighed, it should not have gained any weight. If it gains weight after dipping, the density is below 1.7g/cc. Commercial manufacturers use water only when pressing. The solubility of potassium nitrate in water is an essential aspect of the compaction process, and I don't mess with that. 

It's only recently that the idea of using alcohol when making black powder was introduced. It was alleged that water is the enemy of black powder, and that good black powder could be made by granulating with straight alcohol. After many people made many attempts, the general consensus was that straight alcohol was no good. Then they started diluting the alcohol with more and more water, for their granulated powders, and the results improved. I tried pressing a puck with straight isopropyl alcohol to dampen, and it steadfastly refused to consolidate into a puck. I added that last part for no particular reason, not to make a point. Although, it did show me the importance of dissolving (lubricating?) some of the nitrate.

If I press a nozzleless rocket with 2 1/2% water, the propellant grain will take on the dull gray appearance after a few minutes. After a couple of days of resting, tiny crystals of potassium nitrate can be seen jutting out into the core. I don't worry about them. Since commercial BP manufacturers use 3000-3500psi when making a press cake, I use a similar pressure when pressing damp BP into a rocket tube. Once one tries water at the right amount and right pressure, they will see that it works very well.

I found that knowing the amount of water used, knowing the force used when pressing, and knowing the density of the puck were important to getting perfectly consistent results.

[Richtee]

I have weighed, measured and calculated my densities. They are 1.6-ish. Mechanical compression. Just to add a data point.

 

Perhaps I’ll try straight water. And perhaps... I don’t really have a problem. Maybe I observe too much