Tag cleaning and airlift

Monday 22.8.2016

We actually did some useful work already on the 2nd day of diving. Our task was to located tag attached on top of the amphorae, and clean them up with a sponge - just the same yellow sponge with green rough bottom that everyone has in their kitchen to rub pots and pans. I found a dozen tags or so, and one loose tag that I collected. We kept good bookkeeping on all tags that we found and cleaned.

It seems to be a big problem at nautical archaeological sites, that tags either become unreadable, or get loose from the finds. Good tags cost serious money, and even they can become corroded in just a few years of time. You can not use just any material to tie the tags to finds. You need to consider that the ties must not cause corrosion in finds, or that they do not hamper other research by, e.g., introducing metals to the site, etc. On the other hand, lost tags cause plenty of problems in research, and locating, checking, and cleaning them takes also lots of resources and very valuable time. If someone would invent "forever" (even 10, or 30 years) tags and ties for them, there would be serious money to be made in nautical archaeology circles, because cleaning and replacing tags in underwater sites is also very expensive. But, of course, that money is paid by the next project and not this current one...

Bottom time was 20 minutes (with air) and max depth about 43m. Deco was 2 minutes at 12m with air, 6 minutes at 9m with 80% oxygen, and then finally 12 minutes at 6m with pure oxygen. Divers are prompted for their maximum operating depth and bottom time once they advance to the 9m stop, after which they can proceed upwards only when prompted from topside. This is very conservative and safe schedule. I had my own computer set up time to work normally (with gradient factors) this time, and it cleared after the first few minutes at the 6m stop. So, we did something like 8-10 minutes of extra decos. Either way, this is how we do all dives in future during this project. It feels very safe.

We had four divers in the group and we all do the same decos at the same time. If one member (or pair) of the group stays longer than 20 minutes in the bottom, then the whole group will suffer the consequences in longer decos. You would hear from that a long time.

There is a safety diver attached to each group. He monitors us from the surface during the whole dive and especially carefully during the decos. For possible emergencies, we have the rib always available. Today the rib had to run an errand (pick up the fixed airlift head from shore), and dive operations were stopped until the rib came back.

We also helped to put together the other airlift. Airlifts are like giant vacuum cleaners that can be used to suck silt and other materials to access the finds and to expose whatever is underneath. The "engine" is a vertical rigid tube that sits somewhere on top of the dig. Air is pumped to the bottom of that tube, and rising air creates significant water flow towards the surface, and that creates suction. In our case, the tube is some 30m long and 15cm wide. A flexible "hoover" hose is then attached to the bottom, and with that the suction point can be "easily" moved to proper location. Just like at home you have flexible vacuum cleaner hose to reach all all dusty places.

The airlifted material is pumped to a large sieve, that collects all small finds, in addition to any other larger stones and solid dirt. Solid material falls eventually into a "sieve bucket" under the large sieve, and the bucket can then be detached and brought up to the deck to be searched for finds.

Airlift is put together from shorter components and laid down horizontally before it is sunk. There is a diver at each buoy, and they release their buoys from the tube in fixed order, the furthest one first. The air hose from the compressor is already tied to the tube. The tube ends up floating upright, and then you attach the sieve on top, compressor to the air hose and anchor the airlift at proper place in the bottom. 

We were kind of late coming back to the accommodations, and there was not much time for coursework. This time we used only the measurements done yesterday to draw up the measured site on paper. It turned out that we had done all the usual mistakes that we were originally set up to do. The finds were roughly at correct locations, but their shapes were often funny. I guess the idea was that you learn best from your mistakes.