I have all my supplies out side, a few times my vermiculite has gotten wet. I have used it wet and dry and not noticed a huge difference in the end result. Made some benches and fairly good sized projects out of it.Can't wait to see a pic of your project.
Cindy

RD --- There are two opposing schools of thought on this subject. One school recommends pre-soaking vermiculite for a couple reasons. First, it is so light it can be difficult to mix in uniformly. Second, they feel that any aggregate or added material that is inclined to absorb and/or hold water should be at least pre-dampened to prevent it from rapidly drawing too much water away from the cement and interfering with the cure. The thinking is that because they suck up & retain a given volume of water within themselves, the volume they carry has little or no impact on the overall mix other than preventing the aforementioned "wicking" of water away from the cement. Conversely, aggregates such as sand that are essentially "waterproof" and only hold moisture on their surfce, will rapidly transfer that moisture to the cement altering the actual water/cement ratio of the overall mix.

Then there's the "other" school". A lot of commercial/bulk producers are of the opinion that vermiculite should only be "dry" mixed as the final ingredient before any water is added to the batch. They say that vermiculite is prone to "clump" if it is wet and never achieves uniform distribution throughout the mix...which is the principal reason the first group says you should pre-wet it!

Of note though, is that the bulk mixers state that "hand mixing will remedy any unequal distribution". It may simply be a matter of how the actual mixing process/equipment interacts with this particular combination. So, the way I read it...if you are hand mixing, pre-wetting can probably be beneficial. However, if you are running it through large volume bulk mixing equipment, it may be a hinderence.

You know how it is when you try to get "experts" to agree on anything. As so often is the case, the best bet is to test it out based on your own proceedures and go with what works for you.

You understand the issues so you should be able to make it well either way. I mix it dry, I find it easier to get an even distribution without clumping. Hand mixing wet ingredients is just not practical. Once I have added water, I leave it to set up for five or ten minutes and then may have to add more water because the vermiculite soaks it up. Once the vermiculite has got its fix then it is a source of water for the curing.

One thing about what you said "you don't want it to dry too fast". When curing concrete you don't want it to dry at all, at least not until you are done curing. Concrete will cure under water, in the rain, or under a sprinkler. It continues to get stronger until you let it dry out, then it stops getting stronger.

Sorry, I posted photos elsewhere, forgot about this thread...

Here is a link that might be useful: thread with links to photos.