Name of stuff

I what context?

Clay or Plasticine may meet that criteria

I thought that was a definition of plasticity - permanent deformation (in contrast to elasticity, where after the force is removed, the object returns to its previous shape).

There is lots of stuff will do that. Described as being plastic or malleable.

Plastic

Plastic putty, moulding putty, plasticine, silicone putty or anything on the following list

AJH

Merry foam type of stuff, but one does need to be careful as a lot depends on under what conditions you want this to happen and the environmental constraints etc. Brian

That is the stuff I was thinking of,but Plasticine may do.

The problem is a close coupled toilet and the gasket keeps leaking,I used a commercial one that I was told was the right one but used a bit of silicone and the gasket slipped sideways and leaked, I then used a bigger bit of the same thickness but it leaked, If I had a substance that would stay the same after I removed the cistern I may work out what was going on

Typical Brexiteer. Ask a question you think is sufficient and then accept any answer as long as it fits what you think you want. ;-)

What you were actually asking is of a material that could be trapped between two surfaces ... and then be *measured* to determine the gap.

Had you said that you wouldn't waste peoples time thinking you were looking for a permanent solution or a scientific answer.

So you knew what you wanted it for in the beginning but didn't think to mention it here with yer question?

In light of the above I recommend you get a professional in to do it for you.

Cheers, T i m

Plumbers mait.

Essentially a putty designed for exactly this

Consider also the leak may be the seals associated with the studs/nuts used to tighten the two items together.

Putty& paint is traditional. Paint the surfaces first then fill the gap with putty & then paint the putty. The paint stops the putty from going hard & brittle.

Thanks for that sarcasm, Plastigauge was what I wanted to know Because of vague memories, It turned out not what I needed but Knowledge is good thing which I would not have had were it not for asking.

Not by the evidence.