This is how I made myself a German Parseval-Sigsfeld observation balloon from scratch. The balloon itself is made of polyurethane foam, the rigging from iron wire and the basket from an eraser. I am pretty happy with the result, even if the foam surface is rather coarse och full of small holes and "craters".
And a question: How do you make "How to" articles?
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First make your model of the balloon. I made mine from a paper tube (from a toilet paper roll), two lightbulbs that fitted perfectly into the ends of the tube, covered it with a thin layer of polymer clay.
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Lastly I attached the "sail" made from clay.
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After the clay hardened (my clay made to harden in the oven after 10 minutes at 100 degrees Celsius), I covered it with petroleum jelly and put the balloon on its side in a plastic box filled halfway with plaster, so that exactly half of the balloon was submerged. After the plaster dried I covered the top area of it with petroleum jelly and filled the rest of the box with plaster so that it completely covered the balloon. Put something in the same diameter as the tube from the bottle of foam you are going to use on top of the ballon so that it creates a canal for the foam to enter the mold After the plaster dried I took the mold out of the box and split it in half. Take out the balloon model, clean the mold if needed and use petroleum jelly on all mold surfaces.
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I put the halves back together and taped it firmly so that it wouldn't part when filled with foam.
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I filled the mold with foam through the canal I made. It will spill over a lot, so be on a plastic bag or something similar.
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Mold split open with a finished cast.
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Cast cleaned up. This is my second balloon, that will be painted yellow. I cut off a couple of centimeters of the handle from a small paint brush that fitted in the balloon altitude pegs and attached it to the balloon.
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Iron wire used for the rigging.
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Rigging glued together using super glue.
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Balloon basked shaped from an eraser.
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Basket attached to the rigging.
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Rigging painted.
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"Sails" attached to the balloon and markings added.
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Balloon in German service, attacked by the Entente!
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/Jörgen
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