Can a salt have crystal water and still not be hygroscopic?

[Potassiumchlorate]

They claim that that is the case with copper oxychloride. Blue stars made with it seem to absorb water from the air, though, and the copper oxychloride is the only compound that can come in question. In a few months it increases in weight with almost 1% despite being stored in ziplocs.

 

This summer was also probably the most moist one I ever experienced. It was rarely below 85% relative air humidity

[Mumbles]

I'm not 100% clear what you're asking, but yes certain chemicals can become saturated with water and no longer absorb any from the air. It is also completely possible that water is just settling on the surface and is not chemically being integrated (adsorbing). Ziplock bags are not air or moisture tight, so some could diffuse in over time.

[a_bab]

The very first water - crystalized substance, non-hygroscopic that comes to mind is calcium sulphate, also called gypsum.

 

Depending on your star compo, certain ingredients may become hygroscopical, sometimes down to deliquescence; an example would be sodium bicarbonate.

[Potassiumchlorate]

There are just two salts in it: potassium chlorate and copper oxychloride. Potassium chlorate is not hygroscopic. I avoid all hygroscopic salts except strontium nitrate.

[Mumbles]

Organics most certainly can be hygroscopic as well. Lactose forms hydrates, and most other sugars will absorb water out of the air to at least clump if not form formal hydrates. I wouldn't really worry about 1% unless you're noticing drastically diminished performance. Stars will equilibriate with the surroundings naturally. If you really want to keep them dry, you'd be better off storing them in something that is actually airtight with some sort of desiccant.

[Potassiumchlorate]

There is no lactose in them either. Only organic substances in them are shellac and SGRS. Hm, wait, maybe the shellac is what absorbs/adsorbs water.

 

But in any case I think you're right. They are chlorate stars, so they are easily ignited, and a little bit of water will probably just increase the flame envelope as it evaporates.