Ken Adams

One of THE biggest challenges I face when using the balloon casting technique is applying the correct amount of clay slip on the surface of balloons.

I had always observed an uneven expansion/contraction of balloons on inflation/deflation because it appeared to play HAVOC with the eventual distribution of porcelain slip (semi-liquid clay) across the latex surface. So I decided to precisely measure the expansion/contraction properties of a 16 inch, round Cattex balloon. Note to self: it is possible that balloons made by other manufacturers will behave differently, as may different sized balloons by the same manufacturer!

I most frequently apply slip after an initial, partial balloon inflation, after which I fully inflate and then deflate to varying extent (depending on the texture I wish to develop). Alternatively I apply slip at full inflation, after which I also partially deflate to varying extent. So I began my investigation by using a felt tip pen to mark a succession of 1 centimeter (cm) marks from the apex of a partially inflated balloon (with a 21.5cm diameter), downwards, as well as a single 1cm mark to the right of this vertical axis (to measure horizontal movement). I then fully inflated the baloon to a 37.5cm diameter and measured the distance between all these marks; I then deflated back to 21.5cm diameter and measured everything again.

Chart 1 below shows the extent of expansion when inflating from a 21.5 to 37.5cm diameter along the vertical and horizontal axes of the balloon: the most dramatic movement (>100%) takes place within the top 9cm. That means MUCH MORE slip has to be applied to this zone than anywhere else before inflating fully. If I do not do this, the base of the vessel will be too thin at full expansion, will dry too fast, creating a base which is often far too weak to bear much weight and so at risk of shattering after firing.

Chart 2 shows the extent of balloon contraction when deflating from 37.5cm back to a 21.5cm diameter. The greatest amount takes place within 23.3cm of the balloon's apex, which means that if I am applying slip at full inflation I must spread SIGNIFICANTLY LESS slip in this zone compared to other areas. Otherwise the base of the vessel will end up too think, and possibly even shear and fall off the balloon when carrying out the final deflation!

Chart 3 measures the change in the distance between markers before inflation and after deflation. The diameter of the balloon is the same (21.5cm), but (perhaps unsurprisingly) the effect of inflation had a lasting, distorting effect on the balloon's shape. Areas that stretched the most during inflation failed to contract back to their original state, while those that expanded least actually contracted!

So, this goes to prove that there is a LOT MORE than meets the eye when using the balloon casting technique! It is not just a case of slapping some clay slip on the balloon! But armed with the above information, I am sure you will agree that I (and indeed you) are far more likely to so a good job.

Go on, GIVE IT A GO!