The bronze doors on the Passion façade made by Subirachs, the ones most recently placed on the Nativity façade made by Etsuro Sotoo (who manages the Temple’s team of sculptors) or the bronze crown that was placed on the highest point of the dome of the western sacristy in March 2016, are good examples of the application of the lost-wax casting technique.
This last piece culminates the dome’s symbolic meaning. The sacristy is where priests get dressed before and after services, and Gaudí designed it taking into account two representative ideas of priests’ role: on the one hand, the martyr aspect, of exemplifying sacrifice and, on the other, the ecclesiastic aspect, as pastors or guides who care for and select the grapes from their vineyard. That is why the crown has two parts: on the lower part, there is a ring that represents Jesus’ covenant with humanity and, right above it, is the Crown of life, symbol of the resurrection of the martyrs.
In order to make this piece using the traditional lost-wax casting technique, it was necessary to follow a complex process that included several attempts with different materials. The goal of this sophisticated system is to obtain an empty sculpture in order to use less metal and produce a lighter sculpture.
The first step was a digital design, which became a plaster model thanks to a 3D printer. Then a denser polyethylene –a kind of white cork– model was elaborated with which a crown that could be disassembled in pieces was obtained. Later, the pieces were covered with silicone, an elastic material that hardens quickly, which facilitates de-moulding, in order to produce moulds. That’s when the lost-wax casting technique could begin: the moulds were filled with liquid wax, which sticks to the walls forming a different thickness according to the time it’s kept inside. Then, the wax models were put in drums, embedded in a kind of refractory mortar material. Finally, when the drums were heated, the wax melted, leaving an empty space behind. This space is then filled with liquid bronze by pouring it into each drum.
You can see the lost-wax casting technique that was followed to make the bronze crown of the sacristy in the following video: