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Ooooh, i love this idiot! It overlaps my historical leather working hobby with my hatred of pseudohistorical bullshit.

So, a 30m diameter sphere contains 14.000m3 of gas. Every liter of hydrogen lifts 1.14 grams, and for helium it's 1.05 grams. So 14.000m3 = 14 million liters, or about 14 million grams, which is 14 tons. Sounds pretty decent until you realize that animal hide isn't exactly weightless.

A tanned cowhide, before it's split, is over 4cm thick. Vegetable tanned cattlehide weighs something like 40 kg for a 5m2. But lets see how thick out balloon is. If it can lift 6 tons, it can only weigh some 8000kg itself. Our 30m sphere has a surface area of some 2800m2, so that means our animal hides can weigh, at most, 8000/2800=2.8kg per square meter. Those at home have probably noticed that 2.8 is a bit less than the 8kg you need to use cowhide.

So, what kind of leather or hide were those balloons then? Well leather is usually sold in "ounces per square foot" (even in Europe), so we can just look it up. 2.8kg per square meter is about 9.2 ounce per square foot (says wolfram alpha), and a handy table I have printed says that's around 3.6mm thick.

So, we are too believe that the Egyptians made a 30m balloon, out of what is basically cheap-belt and saddlebag leather, somehow got that not just air-tight, while being ridiculously thin, and then used it to build massive structures.