A "Riff" to a comedian is taking an idea and pushing it until it spins off into something hilarious.
Steven Wright: "The other day I got my house key and car key confused; put the car key in the house lock. It started right up.
So I jumped in and drove it around the block. A cop tried to get me to pull over.
I put my head out the window and yelled: "Get outta my yard!"
That's really riffing.
"Riffing Clay" is doing the very same thing.
Well, it may not be Exactly hilarious, but you know what I mean.
I like to take a basic form and push it. Not only does it bring a new form idea into being, it gives new life to it.
Here's a basic rectangular slab-rolled baking dish that was oooched into a flowing form. (I'm sure oooched is a verb.)
Granted, I did not start with a square base--more of an old TV screen shape; oblong with rounded corners. I measured the circumference of the base with a piece of string, then cut out the side from one very long piece of slabbed clay. (only one join) I attach walls with vinegar water, slathered on with a brush, and compressed the wall base into the floor. This must be done with a very light, form-encouraging touch. Otherwise, the wall will be weakened and not stand up well.
I secure the inside wall at the base by using a clay worm, compress and round and smooth the inside join. Then I manipulate the walls. I might form a slight outward bulge at the bottom; a slight outward flare at the upper rim.
A turntable is a good thing to have here. You can work on the walls and easily check how the whole form looks as you manipulate it. I sometimes add clay to create an interest area. Whatever 'feels' like it's working. Depending on how the clay is acting, I either work while it is soft, or wait until it firms up before 'riffing' it. Every piece is different. It is very important for a potter to cultivate a light, clay-sympathetic touch that builds strength into the clay, and not weaken or overwork it.
*Note: Riffed pieces are sometimes really difficult to photograph.