Glyn, If you like, I can send a videotape to you (NTSC format) of the flushing of a wet system. There is a lot of rust in those pipes, and that rust would form part of the spray going on to collections during normal operation of a sprinkler system. The normal operating condition for a sprinkler head is what we would term an emergency. Dry systems are tested before acceptance. The test can be performed with air pressure or water pressure. If a water pressure method is used, dissolved oxygen in the water creates rust in the pipes which will flood out on collections if the heads are tripped by fire (or a cigarette lighter/candle when the heads are within arms reach; and I have seen them within arms reach in museums). Dry systems pressure tested by air contain oils introduced during the pipe threading process which are not removed prior to installation. The foregoing presumes iron pipe/galvanized iron pipe. Copper pipe is a possibility. The joints of copper pipe are sealed with solder which may contain copper as an additive, but is, nonetheless, more other metals than copper. Bi-metalic corrosion does not require water to weaken joints which may fail when called upon to perform. On balance, pressurized systems, properly maintained (and this includes annual flushing to confirm system integrity and remove accumulated rust), using sprinkler heads which produce a fine mist, are the safest. Here, I am speaking as one who has had to go into a collection area with a hard hat and flashlight as a conservator, as an electrician, and as one who has received training as a firefighter. Jack C. Thompson Thompson Conservation Lab. Portland, OR [log in to unmask]