Creating the larger stacked Sculptures - learning from my mistakes.

I was keen to try and make a large piece using white stoneware and had started to use ES40 from Valentine Clays as this had a more coarse grog than the ES20 white stoneware I had been using previously. Given the scale of the pieces I was attempting I knew that a more robust clay body would help the larger ovoids  hold their shape. 

Throughout the course I have made a large quantity of plaster moulds, and I needed to create some extra  ones to ensure that I had a sufficient number of graduating sizes available for use, as these are the starting point for forming the curved slabs for the construction of the ovoids.  Below are examples of these plaster formers.

Below is the largest ovoid that was used for the white stoneware stacked piece, the clay tubes used to connect the smaller protrusions to the larger bases are made from either hollow wheel thrown narrow cylinders or large hand rolled coils that are cut and pierced to create a hollow tube.

Despite following the same process of construction, and mocking up that I had done with the smaller scale pieces, over half the individual forms that were made for this pieces cracked during the bisque firing. The firing was a very slow firing with a 16 hour pre heat to dry out the work and initially I thought this was the prime reason why the cracking had happened. However when I examined the majority of the areas where cracks had occurred, I believe that they were formed during construction because the leather hard clay had been subjected to stresses that it could not cope with.

Below you can see cracks in different areas, some where around the holes that had been pierced in the clay ovoid using a tube cutter, the downward force had created stress cracks running out in several directions from the hole.

Other cracks were along the joining seems between the two halves of the slab, it would seem that repeatedly stacking the forms on top of each other at the leather hard stage had created too much stress and invisible cracks that couldn’t be seen whilst leather hard, opened up once the clay had shrunk and had begun to vitrify. See below image that shows this.

I was aware that the rapid drying would have also played a role in exacerbating what happened here, but the resulting outcome was one of disappointment and frustration. I had lost almost a whole weeks worth of work and time but I couldn’t let  it ‘hijack’ my momentum and impact on my motivation to keep going. So I took a day off completely and came back with a fresh pair of eyes, took away what I could from what had gone wrong and right with the last piece and started again. 

I made the next one using black stoneware, I didn’t stack them to mock up the forms, but used callipers instead to work out the positioning of each element, I marked the holes out gently then cut with a fine blade rather than pushing in a circle cutter. This time the forms came out fully intact after there bisque firing. See below

Below are the forms stacked prior to the glaze application, a white crawl glaze will be used on this piece.