3D printing, as additive manufacturing is popularly known, has now made its way into the manufacturing of ceramic products. Additive processes are primarily used to produce technical components that cannot be manufactured using conventional ceramic forming methods due to their geometric complexity. When the SKD’s Porzellansammlung (Porcelain Collection) enquired at Fraunhofer IKTS whether methods of this kind could also be used to restore historical works of art, this thus immediately piqued the interest of the workers involved in 3D printing at the institute.

Under the ZIM funding initiative run by the Federal Ministry for Economic Affairs and Climate Action (Grant Number ZF4076454AG9), a 3D porcelain printing method was to be developed for restoration work and applied to a suitable exhibit at the Porzellansammlung. The item selected was an elephant’s trunk, which had been missing from its owner’s head for more than 200 years. Alongside another elephant with an intact trunk, the pachyderm adorns a vase made at the Meissen Porcelain Manufactory in the 18th century.

Out of several additive manufacturing methods that are suitable for ceramics, a lithographic process was selected that experts call “vat photopolymerisation”. The first step in this method is to stir the porcelain powder into a special synthetic resin. One characteristic of this synthetic resin is that when various additives are admixed, and light of a certain wavelength is shone on it – in our case blue light with a wavelength of 452–465 nm – it hardens like the synthetic resin fillings sometimes used in dentistry. Small quantities of this relatively liquid mixture containing ceramic powder are poured into a glass vat that rotates under a fixed recoater blade. This rotating movement spreads a very thin, uniform layer of mixture over the bottom of the vat. The build platform itself is positioned above the vat. The item, hanging upside down, is built up on the platform layer by layer. The platform moves down and dips into the thin layer of mixed powder and synthetic resin. A digital light processing (DLP) module shines light on the section of the layer currently being built, rather like a projector. Within seconds, the synthetic resin hardens, creating a new layer of the component that is just a few micrometres thick. The build platform now briefly lifts above the glass vat, which rotates again under the recoater blade to apply a new layer of powder and synthetic resin. The build step is then repeated and the next layer of the component is cured. Thus, the desired component is created out of thousands of layers. Depending on the height of the part, the build process can take anything from hours to days.

The part that is created by the end of that time is, however, not ceramic, but a plastic object with a high particle content, and thus not the ultimate aim of the restoration work. This object, known as a green body, now has to be cleaned thoroughly to remove any lingering traces of mixed powder and synthetic resin that have not been cured. The synthetic resin is then removed from the part. This step in the process is called “debinding” and involves thermally decomposing the synthetic resin by heating it extremely slowly in a furnace, maintaining the shape of the part. This produces a “white body” that is free from binder and, as with all ceramics, must be fired – or “sintered”, as it is known in technical ceramics. In the case of porcelain, this firing takes place at between 1200 and 1250 °C. During the firing, the ceramic is compressed and acquires its final properties. The volume of the part can drop by as much as 40–45 %. This shrinkage also points to the main difficulty encountered when using 3D printing to restore porcelain objects.

These support structures were kept in place when debinding and sintering the part, so that the trunk could be placed in a good position in the furnace chamber without being distorted by its own weight at a high temperature. After firing, these thin supports were removed. Conveniently, they were attached at points where they were not visible to anyone looking at the work of art. It goes without saying that items of porcelain cannot be taken from the collection and brought to a technical centre so that the replacement part can be adjusted to fit them. For that reason, the original elephant’s head was scanned and a copy also created in plastic using a 3D printing process. This plastic part was produced both in its original size and enlarged by the amount by which the ceramic would shrink. As a result, after the 3D printing and firing, trial adjustments could be made using the replacement part.

The restorer, Heike Ulbricht, could attach the replacement part, now free of any defects, to the historical vase. A historical porcelain item had thus been successfully restored using additive manufacturing based on the original material. Nonetheless, the process still requires further development. For example, the trunk is unglazed; it has simply been coated with white paint. The glazing step poses additional challenges relating, for example, to the colour not matching the original, or the fit at the point of a break. The replacement part selected for the project described here was also relatively small in size, with a small broken edge. The build space in an additive manufacturing system places tight limits on the size of the part. The larger the dimensions of the broken edge, the greater the risk that any slight distortion of the part during firing will be noticeable as a relatively large discrepancy, adversely affecting the fit of the finished replacement part.