Learning to Cast Without Hiding the Seams

Jesmonite, Material Behaviour and Early Limits
February 2025

By February, the research had moved beyond drawing. The early join designs developed through sketchbook work, archival study and digital translation raised a practical question that could not be answered on screen: how would these forms behave when cast?

To test this, I booked onto a two-day course, Casting & Sculpture in Jesmonite, with Dr Sarah Fortais. The casting course I had originally planned to attend was fully booked, and rather than wait several months, I chose to work with the opportunity available. This decision reflected a broader approach within the project: progress mattered more than ideal conditions.

I did not attend the course to collect techniques in isolation. I arrived with a specific structural problem to test.

Entering Material With Intent

I brought with me a simple wooden prototype for a magnetic join — a two-part form designed to lock modular sheets together. At this stage, the form was intentionally basic. Its role was not to resolve the join, but to expose weaknesses once weight, surface and material resistance were introduced.

Jesmonite appealed as a testing material because of its architectural presence. It carries weight without the density of concrete, accepts pigment directly, and records surface detail clearly. It also behaves differently depending on mould type, mix ratio and finishing process, making it a useful material for understanding tolerance and failure early on.

Learning How Materials Push Back

The first day focused on understanding the behaviour of different Jesmonite systems and experimenting with mould-making techniques. Working with polypropylene sheets highlighted how small decisions — fold lines, seams, surface tension — have disproportionate effects on the final cast.

Colour mixing emerged as a critical discipline rather than a decorative choice. Pre-mixing pigments into the liquid component (AC100) before adding powder allowed for far greater control. This reinforced an early principle of the project: colour must be treated as part of the structure, not an afterthought.

Alongside this, a small silicone mould was made in preparation for casting. Careful attention was paid to sealing, release agents and fixing the form to the base — processes that would later prove essential when developing more complex moulds independently.

Casting, Finishing and Surface Decisions

On the second day, the focus shifted to casting and finishing. Using silicone moulds, we explored laminating Jesmonite to produce thin but strong castings before moving on to larger moulds designed to create hollow forms. These processes foregrounded questions of strength, thickness and edge resolution — all of which are critical when thinking about joins that must withstand repeated assembly and disassembly.

Finishing processes were as instructive as casting itself. Rasping, sanding and water polishing revealed how quickly surface character can shift, and how easily a form can lose its architectural weight if overworked. We also looked at sealing for durability, particularly in relation to exterior use.

Time constraints meant that acid etching — a process I am keen to test — was not explored fully during the course. Rather than seeing this as a limitation, it clarified a next step for independent experimentation.

What This Stage Revealed

More important than any single technique was what the course revealed about material behaviour. Jesmonite does not forgive indecision. Small inaccuracies in mould-making or mixing are immediately visible, particularly in forms intended to align precisely or house embedded elements such as magnets.

The course also reinforced the value of working economically. Techniques for recycling waste material — including reusing set Jesmonite powder and incorporating cut-up silicone into new moulds — aligned with the project’s emphasis on reuse, adaptability and material responsibility.

What This Stage Established

This stage was not about mastering Jesmonite as an end in itself. It was about understanding what the material would and would not tolerate when asked to perform a structural role. Casting introduced weight, resistance and surface into the project — factors that drawings alone could not address.

The outcomes of this phase directly informed subsequent decisions about join geometry, tolerance and scale. It also made clear that mould-making would become a critical pressure point in the project, requiring precision, patience and a willingness to abandon forms that did not behave as needed.

By the end of this stage, the project had moved decisively from speculative design into material reality. The joins were no longer ideas to be imagined, but forms that had to survive contact with gravity, force and repetition.