The ingots are very pourous and look like as they were built of styroform (a lot of little bubbles I think [really, a lot, like a tenth of mm in diameter]). I bought the Al on a scrapyard, but in cylindrical bars, so I thought it is clean. On the same scrapyard I bought the crucible (an iron pot) and bulit a brick furnace powered by coal. The clay I use I mine myself next to my house. I know it sounds catastrophic, but I possibly wanted to stick to the ancient methods of forging. In my opinion, gas in the problem and I have no idea about "degassing".
Some of it also sometimes turns into a kind of powder when I hit it with a stick just before melting. I only wonder, why, on low temp, I somehow managed to flattern an Al bar with my hammer, without breaking it. It became 2 times flatter but same dimensions. Then when I forged it, it was perfecly forgable and I even managed to create a simple spear head. However, after making a "sword base" case, I wanted to flattern it and it just cracked into pieces upon the forst hit. Conclusion: if I have a small enough cast, like an ingot and I manage to flatter it, decreasing the volume 2x it becomes forgable... WHAT IS GOING ON?