Sandblasting talk

Yesterday I tried to blast but the nozzle kept clogging. After clearing the clog about 10 times, nothing would come out of the nozzle, so I gave up for the day and later did a visual inspection of the bottom tank valve where the sand and air mix. The mixing valve was totally packed with sand, but I think what was causing the clogging were these large flakes of hard, gray material that were mixed in with the sand. I have no idea what those flakes were, and I never put them in there. Maybe they were some kind of reaction between the sand and the inside of the tank? I cleaned the tank and the mixing valve thoroughly, got rid of all those gray flakes and filled the tank with clean sand. I also tried opening the mixing valve only after opening the blast valve (i.e., keeping the mixing valve closed until after opening the blast valve). This seemed to work very well. I got a lot of blasting done today with no clogs whatsoever. I am still wondering what those gray flakes were, though.