For years my habit when a wash went wrong in front of students was to stop, put the board face down, and start a fresh sheet. Clean, decisive, and I realise now it taught them absolutely nothing except that mistakes are terminal.

This past Saturday I was demonstrating a leaf of Berberis, and the first wash of sap green went down too fast. It pooled along one edge and left a hard tide line where I lifted the brush a beat too soon. Nine people watched me do it.

I left it there. Then I went back in with a barely damp brush and worked the tideline outward while the paper was still at that stage where it will forgive you, which is a window of perhaps forty seconds and no longer. It dried flat. Not invisible, but honest. The leaf still reads as a leaf.

I told them what I was doing while I was doing it, which I have never done before, and one woman at the back said she had assumed that once a wash was down you simply lived with it. She has been painting for eleven years.

The correction is now the part of the class people ask about. I have mixed feelings about that, since a demonstration of doing it right the first time would have been more useful, but it was the truer thing to show.