Pumped stars crumbling

[insutama]

I have very limited Experiance with making stars so far I have made 2 batches of glitter stars with mgal and they worked great with 12% water dried nice and hard now I tried making some tt stars using around the same amount of water and when they dried about 20% of them crumbled when I tried to pick them up. is this because I didn't use enough water or is there others reasons that could cause crumbly stars ?

[db5086]

How long have the been drying. Charcoal rich stars can take a bit longer to dry completely. Put them in a warm and sunny spot for a few days and see if they harden up for you.

[insutama]

Day 4 today I have them in a drying container full of calcium chloride

[nater]

How much pressure did you use when pumping? Did you use a single star pump or a plate?

 

The culprit could be not enough water, not enough pressure or if the comp could be drying out by the last stars you made and crumbling.

 

Tell us more about how you mixed, dampened and pressed these stars.

[schroedinger]

Not Enough water. Even when pumping TT you will need about 17 %, and for cut up to 30% depending on your charcoal (normally about 20-25%).

[insutama]

I have a 1/4" star pump and I pick up the star comp in it then I compress it down on a steel plate then smack it with my rubber mallet 3 or 4 times then pop it out on a drying screen.

I wet the composition by putting it on a scale and spraying with a small pump spray bottle little at a time and mixing and when I reach 12% the mix is sticky enough to start staying in the pump and compressing so I stopped adding water and I think your right about the Comp drying up by the time I'm pumping the last stars and also seems like i need more that 12% water to begin with thanks for the advice

[schroedinger]

If you have problems with the comp drying out. Use a tub with an easy to remove lid or an lid with a hole in it to reduce evaporation.

Also with high charcoal content comps like TT it is good to sieve the comp during wetting and let it sit for some time, since some charcoals absorb water over time.

[insutama]

Ok sounds good I'll try this what size screening 20 mesh?

[Mumbles]

The size doesn't matter much. I use a 12 or 16 mesh screen, whatever is handy and not too fine.

 

Also, putting them in an aggressive drying environment too soon, like CaCl2, can cause them to become driven in. They'll appear dry on the outside, but still be wet and soft on the interior. Breaking a star in half and scratching it will tell you. Dry TT should be hard, uniformly dark grey, and leave a white streak all the way through the scratch.

[insutama]

thanks for clearing that up mumbles ya usually i let them dry for a couple days on a screen with a fan then after a couple days i put them in drying container with cacl2

[Sparx88]

Yah charcoal based stars do take about twice as long to dry completely+. Also letting the patty marinate in a plastic baggy sealed and air pressed out for an hour or two will help with that. Lets the nitrate to be more absorbed by the charcoal and will fully activate the dextrin for harder stars.

[insutama]

thanks guys

[schroedinger]

Also if you are on the run, putting the mass into a vacum chamber greatly fastens up the wetting process.

[insutama]

im sorry i dont follow what do i do ? normally after i get about 12 - 15% water i cover and let sit for a hour then i pump is that what your talking about ?

[schroedinger]

No. Letting it sit for some time is an easy a very good way. But if you put a just wetted mix into a vacuum, air is sucked out of all the small holes and water can penetrate faster. It can speed up the process in case you have nothing else to do during the time you wait for the mix.

[insutama]

Ah gotcha good to know

[TYRONEEZEKIEL]

High charcoal content stars such as TT or willow etc. are VERY susceptible to fracturing out of the press simply because they are very fragile by nature. Im pretty sure that its not a lack of moisture or too high of pressure. In both of those cases you will see hairline cracks through the stars out of the pump as they expand from the release of pressure.

 

The solution to use here is when wetting the stars, use a gum arabic solution. Dissolve gum arabic into your water before pumping. Use 4g gum arabic per 100g water. This will not affect your burn time, spark production, or any other characteristic of your stars in any way. It will make your stars much less fragile during the drying phase.

[oldspark]

I use 5% water for 1/4 inch pumped stars and they dry hard, pretty sure 5% is what is recommended for pumped stars, I did not pull that figure out of a hat.

Edit-I might be wrong about the 5%, I see Ned's article says use about 10%.

I press at 2000 lbs so dont need much water.

Edited July 14, 2015 by oldspark

[nater]

You are not wrong Oldspark. 5%-10% is good range for pumping stars, some times more and others a little less. The comp itself, pressing force, weather, and any moisture in your individual chems all influence how much solvent needs to be added when you press the stars.

[Mumbles]

To get into the 5% realm, you normally have to be exerting some serious pressure. I normally use between 8 and 12% when hand ramming or compressing with something simple like an arbor press.

 

This is purely speculation, but I wonder if you could get away with less by using a predissolved binder like dextrin solution, gum arabic solution, or thin wheat paste. I would think that if less of the water is wasted dissolving nitrate, or soaking into charcoal, that less would be required for adhesion.

[nater]

True, I do use a hydraulic press and often bind using a dextrin solution rather than including dextrin in the comp. Comparing the forces needed to properly press an item using a dextrin solution or including the binder in the comp would be a worthwhile experiment.