A project to capture and reproduce an 18th-century coin from the collection of Michael Talbot, Associate Professor in the History of the Ottoman Empire and Modern Middle East at the University of Greenwich.
The original silver coin, just 28.7 mm in diameter, was transformed into a one-meter-wide replica carved from birch plywood.
Denomination: 1 Budju
Material: Silver
Width: 28.7mm
Weight: 10.37g
Mint: Algiers
Year: 1239 (1824)
Language: Arabic
The Arabic inscription reads:
“Sultan of the two lands and ruler of the two seas, the sultan Mahmud Khan, may his victory be glorious.”
Replicating the coin comprised of the following steps:
Capture the Image:
Take a high-resolution, evenly lit, perpendicular photograph of the coin.
The original coin would have been struck by hand, here it has resulted in a slightly misaligned strike that left excess material, or ‘flashing,’ on the lower edges.
Generate a Depth Map:
Convert the photograph into a grayscale depth map – an image where varying shades of black and white represent the coin’s relief (depth and height).
There are a number of techniques for estimating a depth map from a 2d image; traditional, rule-based / algorithmic techniques – and more recently AI neural networks trained on a huge datasets have proved very effective.
Some examples of these (open source) models are Marigold, Midas, DepthFM. There are online versions of these that can be used – but in this instance DepthAnything was used to process the image locally.
Create a 3D Model
Apply the depth map as a displacement map onto a flat digital model to create a 3D surface.
With the depthmap image as a PNG image, this can be applied as a displacement modifier in most 3D editing software then exported as an OBJ.
• Rhino – HEIGHTFIELD command, select the depthmap file. Increase the sample points to get a finer mesh
• 3d Studio – Select the geometry (cylinder) and apply DISPLACE modifier, load the depthmap image – use the strength parameter to control height of displacement.
• Blender – Add new mesh plane, subdivide the mesh in object mode many times, apply the displace modifier, create in that a new texture and open the depthmap image to apply it.
CNC Toolpath Generation
Import the .OBJ into CAM software to generate the toolpath – these are the digital instructions the CNC machine will follow to take the drill bit around the material fixed on the CNC bed.
CNC Machining:
Run the CNC operation to carve the 3D coin shape from a block of plywood according to the toolpath.
The CNC machine will perform a two-pass subtractive process: a roughing pass with a larger bit to remove the bulk of the material, followed by a finishing pass with a smaller bit for finer details.
The first pass took around 40 minutes, while the second pass took 6 hours to complete.
Finishing
Sand down any rough areas on the carved coin and apply Tung oil to finish the wood.
The CNC process can leave fine tool marks, and rough surfaces; sanding smooths these imperfections, revealing the wood’s true character. Tung oil, a penetrating oil finish applied over 3 days, soaks into the wood fibres, making them appear richer and darker. This brings out the grain and enhances the contrast and visual depth of the wood’s natural pattern.
After the Great Exhibition of 1851, the Crystal Palace structure was moved from Hyde Park to Penge Place in South London, now known as Crystal Palace. A railway station and an elaborate subway were also built for it.
The Palace itself burned down in 1936, and the railway station was demolished in 1961, leading to the subway’s gradual disrepair. Around 2010, the Friends of Crystal Palace, Bromley Council, and Historic England initiated a major restoration project. The subway reopened earlier this year.
The vaulted area is directly below Crystal Palace Parade and features 18 stone columns topped by terracotta-and-cream brickwork fanned vaults.
The busy road is about 1.5 meters above the vaults – buses can be seen on the road in this scan.
The same view in elevation
This section is made up of both scanner’s output – the BLK2Go for the closer / colour point clouds and the Hovermap for the longer range street captures
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.
