This is an undertaking to replicate an 18th century Ottoman gravestone which is currently held by the Royal Greenwich Heritage Trust.
Michael Talbot, Associate Professor in the History of the Ottoman Empire and Modern Middle East at the University of Greenwich, was informed by the Royal Greenwich Heritage Trust about an “Arabic tablet” which he identified as an Ottoman object and subsequently transcribed and translated the inscriptions on it.
The original tombstone itself is a late-eighteenth century artifact made of limestone or marble featuring Ottoman Turkish inscriptions in the sülüs calligraphic style. The gravestone’s origin is not clear, but it was possibly brought back from Constantinople as a memento by a British officer in the 19th century. The inscription features a poetic composition reflecting the youth and untimely death of its owner.
The tombstone itself measures 72 cm x 21.3 cm x 11 cm thick. The stone is broken at the bottom where it would have originally been set into the ground. It would also have been topped with a carved representation of the headgear associated with the deceased’s rank and profession, likely a turban, indicating a position in the religious-scholarly class.
Replicating the tombstone comprised of the following steps:
- Scanning: photograph all sides / angles of the object and utilising photogrammetry to generate an accurate 3d digital model of it.
- 3D Printing: produce actual-size moulds of the tombstone from the scan model.
- Casting: pour Jesmonite (similar to plaster) into the moulds and allow to set
Scanning
Photogrammetry is a process of 3d scanning whereby many photographs of an object are used to create an accurate digital model. Common points in the overlapping photos are identified in order to align them and create a point cloud – a 3D representation of the model as dots in 3d space extracted from these aligned images. This is further refined into a mesh model to form a network of triangles which are lastly “wrapped” with the texture that has been derived from the photographs to provide colour and detail to the 3D surfaces.
To get the best results for use with the photogrammetry software there should be many photos which are sharp, evenly lit – with as little shadow as possible – and capture all sides and angles. Ideally the object would be photographed in a photo-studio with controlled lights and blank backdrop etc, but since this was not possible in this case some soft lighting was brought to site to offset directional shadows from windows.
The model was photographed with a Panasonic Lumix FZ82 (on a tripod) which is a mid-range bridge camera. Manual settings / RAW format, approximately 500 photographs – then post processed in Adobe Lightroom to eliminate any blurred shots and batch edited to further reduce shadows and bump up highlights.
The photogrammetry software used for this exercise is called Reality Capture. Since the tombstone was too heavy to stand up on its end it had to be laid flat, horizontally for one set of photos and then flipped onto its front to capture the other side. Ideally Reality Capture would have automatically detected all the photos as a single object but in this case it generated two separate components: a top and a bottom. To fix this one halve had to be flipped and manually aligned in the software in order to produce a single complete model.
With the model successfully generated it can be exported to a variety of formats for a variety of purposes. For viewing / zooming / spinning the model online it has been exported to Sketchfab this model includes the texture for added realism.
For purposes of 3d printing the model is exported to a common OBJ file format. The texture wrapping step is not important to 3D printing since these printers do not reproduce the model’s colour, so the version used there is effectively monochrome.
3D Printing
Commonly 3D prints are created using PLA or photopolymer resin etc. While these materials recreate accurate models, they can feel light and “plasticky”. For the tombstone it was important to recreate as much as possible the tactility of a stone / marble material and have a weightiness that approaches something more authentic to the original in that respect.
For these reasons instead of printing the model directly, moulds of the model – effectively an inverted version of the 3d scan – were produced, into which a plaster-like material was poured and left to set.
Material used to print the moulds: TPU 95A flexible filament. This means the resulting structure is supple and bendable – strong enough to hold the pour but can be peeled away from the cast once it has set.
3D Printer used: Bambu Lab P1P. The maximum printable area of this printer is much smaller than the size of the tombstone itself meaning the mould needed to be printed as 4 sections and then reassembled for casting. Each of the sections took 18 hours to print. (shorter test section illustrated above with red mould).
Since a full dense model would use a lot of casting material – and also create a very heavy model – only an outside skin of about 10-15 cm of the model was needed to be cast. To achieve this an additional 3d printed core of the tombstone was placed inside the mould in order to cast around it. The final model is lighter and more economic with casting material and retains the proper look and feel – but with a hidden, enclosed, non-dense core.
Casting
The material used to cast the tombstone is Jesmonite – this is a water-based composite material that combines natural materials such as gypsum and resin with various other components, including water-based polymers. It is known for its flexibility, durability, and environmental friendliness compared to traditional resin-based materials. It is also less likely to chip or crack like regular plaster and can be mixed with a pigment to colour the material.
Images below show Jesmonite being mixed with pigments and some experimental, test colours.
The 4 separate 3d printed moulds were taped together to form a continuous mould to be cast into – so the moulds were printed as 4 sections with the casting as a single object which required no reassembly.
The photo above left shows the 3d printed, light core inside the mould while on the right the Jesmonite is poured in to fill the mould with the core embedded inside.
The cast itself sets fairly quickly in about 20 minutes and is then ready to be removed from the mould. The moulds themselves can be re-used to produce more replicas (though the core would require re-printing).
The weight of the replica is initially about a third of the weight of the original, though as the moisture evaporates over a few weeks it becomes somewhat lighter, though still a substantial weight of about 15kg.
