First cast of the concrete tentacle lamp base done. Not quite an epic fail but this one certainly won't see completion. The concrete is powdery at the top which is from too much de-moulding oil (vegetable oil). Thinking maybe I'll juts try using the PVA de-moulding agent for the next one. But worse than that, there's a crack at about a quarter of the way down which goes all the way round. It wiggles so I'm sure with a good pull, the tip will come right off. There is a PVC duct cast into the piece which ends right about where the crack is. I didn't bother trying to reinforce the concrete with wire but perhaps next time I will, particularly around the point where this crack has occurred.
And apart from all that, I used a six month old half empty bag of concrete that I found lying around under the house.
So next time, use fresh new concrete, use de-moulding agent sparingly and add a little reinforcing wire.
It popped out of the mould with no real trouble so it's looking good for multiple casts.