Turning, also known as trimming, is the process of carving off excess clay and giving the bowl an attractive base, known as a foot ring. 

Apart from the aesthetic qualities of the foot ring, the process will even out the thickness of the walls and will make the bowl lighter.  This makes the finished bowl much nicer to handle. It also reduces the risk of cracking when the bowl is fired.

There are special tools for this purpose.

Turning requires the clay to have dried slightly to 'leather hard' consistency. This is normally 48 hours after throwing (but can be less in mid-summer).

The process starts with an assessment of the size and height of the required foot ring.

Then the piece is placed upside down on the wheel-head, centered and fixed in place with clay 'brakes'. As the wheel is spun, two guide lines are etched into the base to mark where the foot ring will be.

I prefer to trim the centre of the base first, and then the walls. (Apologies that the slide show does not show the sequence properly)

I start by using the carving tool, applied with slight downward pressure, to trim out a vertical footer. Downward pressure is applied from the very centre of the base, outwards towards the foot ring. This takes a few passes. and is repeated until the base just starts to give (i.e. bend a little). This way you know you have carved the correct depth.

Then a groove is the outside at the outside of the foot ring (using the guide line etched earlier). The groove is carved deep enough so as to be the same depth as the inside of the base. Then you can use the carving tool to trim off the 'flange' of the base and carved away the walls so that the outside form has a nice continuous shape that meets the foot ring at the correct depth. The idea is to end up with even thickness (of less that 1cm) throughout the form.  The process takes several 'passes' and you will be surprised at just how much clay gets trimmed off.

The walls of the foot ring are then fine tuned and, once you are happy with the shape, the foot ring can be gently smoothed out using fine tools and a damp small sponge.

I will show you the different tools and the ways to hold and apply them.

The exact size and thickness of the foot will depend on the relative dimension and shape of the bowl. To assess the thickness you can gently press the very base of the bowl (inside the foot-ring) which should be just at the point of being flexible.  You can also tap the sides and listen to the sound.