Mould Making

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Week one of our next module began with Terry (the one wearing the beret) and I catching the ferry and Uber to reach the Pt Chevalier studio and Front Room Gallery of Brendan Adams and his lovely partner Kathryn. To reach his studio you enter through the front room (no prizes for guessing the inspiration for the name chosen for his gallery) and are immediately surrounded by his incredible work. Take a look at http://badams.co.nz

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The plan for the day was to create a slip cast plaster mould of a found object that would provide us with an opportunity to work through the process and perhaps begin to formulate some ideas around the concept of Containment. The clay objects produced by our moulds will form the central pieces around which a sculptural work will be made. We will be using white earthenware (to be glazed white or clear) to slip cast our clay objects and red terra-cotta (unglazed or dark oxide finish) clay to form the sculptural container/restraint. Really looking forward to this already.

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The found object settled on for my first mould was the ping pong ball (no idea yet where this will lead). We began with a slab of clay onto which we positioned our objects. Clay was removed to allow the balls to fit exactly half way down into the clay, ensuring that the clay was pressed closely to the sides of the object and that the clay surface was as flat as possible. To provide a funnel into which the liquid slip is to be poured, clay was added in the shape of a cone. This was then surrounded by a wooden frame into which our plaster mixture was poured. It is a good idea to vibrate the liquid plaster before it hardens to help any trapped air bubbles to lift away from the object to leave a clean and smooth surface.

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After the first half of the mould has set, the clay was removed (do not re-use this clay for firing as any bits of plaster remaining will cause your clay to crack or even explode when fired). Three small depressions were made in the plaster surface to provide a key for the other half of the mould to fit, thereby ensuring that when the two halves eventually come together they will be in the correct position. To ensure the two halves of the mould would come apart after the second was poured onto the surface of the first, soft soap was first brushed on, then wiped off (repeat this step 4-5 times to leave the thinnest layer over the plaster and object surface). The mould was again framed up and the second plaster mould poured, vibrated and left to set. Once warm to the touch, the framework was removed and the two halves separated. The edges should be arrised and cleaned of any potential crumbs of plaster that could make their way into your slip cast work when it is being made.

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Day two began with an inspection of our moulds (which had been drying with the assistance of a dehumidifier). Nice and dry, we set about preparing the slip that we would be pouring into our moulds. The slip was pre-made and purchased from our local supplier. With the aid of a drill and paint stirrer attachment, we gently mixed the slip (trying to avoid introducing any air bubbles) and poured it through a sieve into a jug for pouring.

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Slip was poured into the plaster cast moulds using the pour holes created for this purpose when the moulds were made. The job of the plaster is to absorb the water from the slip and in doing so leave a skin of clay that becomes thicker the more time the slip if left inside the mould. To avoid a solid object, the remaining liquid slip is poured from the mould after sufficient time has elapsed to form a clay wall of suitable thickness. In my case, we allowed the slip to sit in the mould for 6-8 minutes before pouring out the excess. The thickness around the pour hole can give an indication of the thickness of the internal walls. With some larger moulds, a vacuum can be created inside the mould when attempting to pour out the excess slip, and if not compensated for, will result in the collapse of the internal walls. To avoid this, air is introduced into the internal cavity by blowing into a flexible plastic tube as the liquid slip is poured out. Left to harden for approximately an hour, the two halves were separated to release the clay objects. Although able to be held gently, too much vibration and a form of liquefaction happens, with the clay becoming more jelly-like. Could hold more possibilities for this generic shape.

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Some preliminary drawings of how my cast sphere might be able to be contained. I am thinking about casting some old rebar to form the connecting components and cage structure for the two ideas above.

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More moulds under construction to provide the component parts for the next steps in creating a work that incorporates the notion of containment in some form.

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There can be a fine line between success and failure. With the two halves of a ball mould successfully constructed, I decided that this presented a great opportunity to use one half of the ball mould to form a hump mould for making small bowl forms etc. After ensuring I coated the plaster surface 4-5 times with soft soap (which I made using lux soap flakes and boiling water until I achieved a brushable consistency) and adding a clay collar to provide for an extended form, I poured the water/plaster mix into this and allowed to harden. When it came to separating the two pieces however, this was not to be. I decided to seek help and asked Brendan if it would help to wait until the plaster dried out further? His reply suggested that no this would not really help and that the most effective method would involve the use of a hammer. After some initial tentative taps with a piece of timber without success, I did indeed smack it with a hammer and wouldn’t you know it, success once again. Let the drying begin (it has already taken two weeks and the moulds are still feeling a little damp).

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Week four introduced another method of mould construction which is used to create a slump mould into which clay can be ‘slumped’ to hold a specific form. In my case, I decided to construct a wake form, as I have a project in mind for later in the year. I am also hoping that using the slump mould, I might also be able to make a ‘hump’ mould over which I might be able to drape clay that will keep its external texture intact. I will of course make sure I coat the surface of the plaster mould well with soft soap to ensure that the two pieces can be separated once hardening has occurred (and perhaps keep my trusty hammer somewhere to hand).

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With my own moulds finally dry enough to begin casting, I poured my first attempts. Quite happy with the outcome although some issues/challenges now obvious. The size of the hole allowed for pouring the ball may take some adjustment before I am able to join at that site, or filling if it may be better to join at another location on the sphere. Drying of the ball to the point at which joining is optimum (another consideration) may require some kind of perforated sling, as previous spherical shapes have slumped during the drying process (handling may have impacted also). The re-bar mould has become mostly solid due to its dimensions and I am a little worried that the potential for air pockets is high. Perhaps threading a thin needle down the centre may alleviate this concern? My mould has also shown me that the two halves must fit together without any gaps as mine chipped a little during construction, and this has allowed slip to egress between the two bars. There is also a gap to the outside edge which is a problem. Will need to recast if they are working well as a form. Brendan Adams gave me a joining slip recipe as follows… 1/3 slip – 1/3 water – 1/3 vinegar. Something about the flocculation? Will explain more when I get my head around this more. Next iteration of this process might be to press mould all component parts. Joining different types of clay and clay forms produced using different techniques does present some risk. Joining a slip cast form with one press moulded, may not be the best approach. Better that all parts are either slip cast or press moulded. Next move will be to try both approaches before going ahead with maquette of my planned projects.

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Some days are filled with experimentation and the inevitable frustration that goes along with the challenges that reveal themselves during this time. Making more decisions today about what NOT to pursue. Pressing component parts produces a less crisp finish and more excess to remove/smooth. I am, however, finding that joining pressed pieces is a little easier than joining those that have been poured. The issues presented by differing clay types and construction techniques is also beginning to frustrate progress. Time for a cuppa.

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Ball mould is continuing to progress, with decisions being made in response to each challenge. Think I might hopefully have one piece of the many I will need to move my planned work forward. Keep going.

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Didn’t get the lengths the correct size to allow the sphere to fit tightly within the cage structure due to shrinkage issues. Thought I would continue construction of the cage to work out the best time to make joins, support systems etc. No decision yet as to whether I will position the sphere within a completely enclosed cage and fire as one piece, or whether this is too complicated and it may be better to glue the final side of the cage once the sphere and cage have been fired separately. Watch this space.

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Test firing of proposed component parts (of a planned larger scale work inspired by Brendan’s ‘containment’ mould making project) has worked well with all joins intact and little warping. Now for more accurate measurements, better allowances for shrinkage and more careful finishing, another cage to be constructed to give expression to the way we can be constricted/constrained by both our physical and psychological environments (and I thought I would be making plates and bowls, or even a nice hand-made vessel for that morning cuppa).

Another slightly larger cage has been constructed in the hope that after drying and firing, the white spherical shape may indeed be ‘contained’ within it.

The slip cast orb has been fired to 1160 and currently fits inside the unfired larger red brick clay moulded rebar cage. Not confident that the sphere will fit once the shrinkage of firing has occurred, so will wait until after assessment day to do this. I am happy with the general effect of this ‘containment’ and am planning to pursue the ‘bigger picture’ next year. Watch this space.

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Brendan makes the most amazing and fantastical teapots and cups. On one of our days spent with him in his studio, we were allowed to make use of his moulds to construct our own crazy pot. This is mine. Currently in the kiln. I have to say I am a little concerned that my brush application of his strontium matte glazes might not produce the outcome I anticipated at the start of this little project.

Happy. With some refinement, this olive oil pot might just make it to the table (truth be told, it is going to grace our table anyway as a celebration of each little success this year). The brushed application produced a reasonably flat fired surface, unlike another glaze I applied with a brush. Of course, many factors come into play when glazing a work. If this year of trial and experimentation is teaching me anything, it is the importance of recording everything and narrowing down as much of the unknown as possible. That way, when something works as intended, or indeed doesn’t, there is every chance that whatever change is necessary can be established and applied.