3D and RTI: Shedding Light on Damaged Ogham

By Dr Megan Kasten, OG(H)AM’s UK Postdoctoral Researcher

In March 2023’s Ogham of the month, I mentioned that OG(H)AM’s application of digital imaging techniques to the Bac Mhic Connain knife handle (S-INV-001; NMS X.GNB 134) has offered new insights into the damaged parts of the ogham inscription. Because this is one of only four known ogham-incised knife handles, we need an accurate reading of the inscription on Bac Mhic Connain to better understand the use of ogham on household objects. The inscription is very finely carved, so during last year’s visit to the National Museum of Scotland we decided that, in addition to recording the object in 3D with photogrammetry, we would also use Reflectance Transformation Imaging (RTI) to allow for closer inspection of the inscription.

A white bone handle sits on a black background next to a red and white scale bar and barely visible black reflective ball for RTI purposes.
A screenshot from the RTI of the Bac Mhic Connain knife handle (S-INV-001; NMS X.GNB 134) with Image Unsharp Masking applied.

Though RTI and photogrammetry use much the same equipment, the two digital imaging techniques provide different benefits to artefact analysis. RTI has been key to analysing very fine inscriptions on small objects like this, especially in damaged or worn areas. RTI is not a 3D imaging technique; it is often described as a 2.5D technique. In RTI, the object and camera are kept in one position, but the light source is moved to a different position for each photograph. In the image above, a black reflective ball is barely visible next to the scale bar – this ball reflects the light’s position in each photo so that the software can determine how the changing light affects each pixel of the photo, and so allows the software to recreate how the lighting affects the surface of the object. A number of digital rendering and image processing techniques can then be applied to the object to bring out different aspects of the carving, like Image Unsharp Masking, which has been used on many of the RTI images in this blog. In the case of this object, these imaging options highlight differences in the character and direction of shallow marks on the surface to help us discern which were intended as part of the inscription and which are the result of damage or weathering.

Photogrammetry was used to create a 3D model of the knife handle; by taking overlapping photographs of an object, photogrammetric software like Agisoft Metashape can reconstruct the geometric structure of the surface of an object. A macro lens was used when recording this object, a lens designed for extreme close-up photography, which meant that, despite the small size of the knife handle and its inscription, most of the lightly-incised inscription was visible in the mesh of the 3D model. However, the relative depth of the damage to the knife handle’s surface is often deeper than the inscription on this object; most 3D shaders that are applied to 3D models to bring out details, like Radiance Scaling, often use relative depth in its algorithm, making it less useful for visualizing the inscription itself than RTI, as seen in the screenshot below. The 3D model does have several benefits over the RTI, including the ability to visualise the overall shape of the artefact (particularly the central ridge the carver used to guide the inscription), and the ability to take scaled measurements directly from the 3D model.

The 3D model of the knife handle is silver and shiny, though this is not particularly useful in reading the inscription.
The photogrammetric 3D model of the Bac Mhic Connain knife handle (S-INV-001; NMS X.GNB 134) with the Minneart shader applied in MeshLab.

Though still not an easy task, the combination of these digital imaging tools helps us to parse the various topographical changes of the knife’s surface. Because the author of the ogham inscription carved the strokes perpendicular to the grain of the cetacean bone, these do stand out from the damaged areas visually, and even more so when the colour of the surface is removed digitally.

Based on visual inspection, Katherine Forsyth (1996) read the inscription as:

M(a/o)QUNTEN( /a)CoT

There are three areas of doubt in this inscription – the second letter, the space between N and C, and the penultimate letter between the C and T. The second letter of the inscription is uncertain because the knife handle has been damaged. However, if we use RTI, we can carefully examine the small area of intact surface for similarly cut strokes as we change the angle of the lighting.

The first area of doubt highlighted with a red box.

In the top right image below, three potential cuts are identified. The two lower cuts suggest that, in fact, at least two vowel strokes occupied this space, proposed in the lower left image. The third, slightly off-set cut above these two remnants isn’t unexpected, as there are other strokes later in the inscription that seem to have been made in two attempts.

Top left: RTI with Image Unsharp Masking applied; Top right: Cut marks that may have been part of the original inscription; Bottom: Two possible interpretations, given the available space and remaining marks.

There is the possibility for a third stroke in this space (as shown in the lower right image above). However, careful measurements taken from the 3D model of this knife handle would suggest this to be unlikely. On this object, the spacing between strokes belonging to a single letter ranges between 0.9mm-1.4mm. The spacing between clearly identifiable letters ranges between 1.1mm-2.0mm. The space between the M and Q is 4.2mm. If the minimum of these ranges were adhered to, this would allow adequate space for a U. However, early in the inscription, spaces between letters often exceeded 1.5mm, which makes it more likely that the two strokes of an O were intended for this space.

A screenshot of the 3D model where the gap between the M and Q has been measured. (Note: The 3D model is scaled in meters.)
A screenshot of the 3D model where the spaces between strokes in the first part of the inscription have been measured.

The RTI also allowed for closer analysis of the space between the N and C, suggested to either be empty space or an A. This part of the knife handle has been damaged in a different way to the leeching and erosion seen elsewhere on the handle, which makes the questionable region more difficult to interpret. While there are three short marks parallel to the other strokes, these are of a different character: they lack the sharp V-shaped stroke and central ‘cut’ of a sharp implement found in all of the other strokes. Instead, they appear to be U-shaped, possibly made with a different tool.

Highlighting the second questionable area of the inscription, which may have been intended as a blank space.

These strokes instead appear to be more similar to an oblique scuff in the same area, pictured below, so it is unlikely that these were intended as part of the inscription. Though the space between the last stroke of the N and the first stroke of the C is large, 3mm, it seems as though this was intentional.

An oblique scuff which has a similar U-shaped nature to the area in question.

In one of these questionable marks, there is a slightly sharper implement used at some point, but this appears to be more closely related to the implement used to create the thicker groove highlighted below. There is also a substantial amount of damage of another nature to the first stroke of the C. After taking all of these factors into account, the relatively shallow and U-shaped nature of these strokes, and the range of subsequent damage this area of the knife has received, it seems most likely that this was intended to be a 3mm space in the inscription by the original author.

Highlighting the sharp implement that was used in the creation of the large groove which appears to have affected the first of the questionable marks.
Showing on the 3D model where the larger groove seems to continue obliquely into the inscription.

The final questionable letter is just after the C, where a significant break has occurred. Because of this damage, measuring the spacing here is unreliable. Just after the break, however, there does appear to be the remnant of a single stroke downward from the ridge the inscription has been following. Because it starts below the ridge, roughly where the other letters belonging to the first aicme have started earlier in the inscription, and because the other vowel strokes are consistently visible above the ridge, it is possible that this was intended to be a B or an L. However, there is very little of the stroke left, and an inconvenient crack above the stroke has caused some loss of the bone surface, so it is also possible that this could be vowel, so an A or O. This single stroke remnant is 2.5mm from the first stroke of the final letter T and, less reliably, about 3mm from the final stroke of the preceding C, suggesting that a B or A is most likely. The size of this space would not be unexpected if the 3mm gap between the N and C was intended to be a blank space, as we now suspect.

The box highlights the gap between the C and T, with the stroke remnant visible just after the break.

Based on these observations, it seems that the inscription reads:

M(o/u)QUNTENC(b/l/a/o)T

With the most likely readings:

MOQUNTENCAT

Or

MOQUNTENCBT

Option A, MOQUNTENCAT, would make the most linguistic sense and is the more likely scenario. Option B would introduce new problems in terms of interpretation, as the ending NCBT is not particularly readable. While there is increased spacing between the N, C, B and T, this can either be explained as the author’s attempt to fill the remaining space as they approached the end of the knife handle (as one would expect in the case of option A), or, in the case of Option B, this spacing could be an intentional way of separating the final three letters from the rest of the inscription, possibly indicating the use of abbreviation or another meaning for these letters.

MOQUNTEN C B T

Though this analysis does not remove all doubt from the reading of the inscription, the strategic combination of digital imaging methods has offered new insights that will help build our wider understanding of ogham inscriptions on household objects.

Further Reading

Armit, I 1992 The Later Prehistory of the Western Isles of Scotland. Oxford (=BAR Brit Ser 221).

Forsyth, K. 1996. The Ogham Inscriptions of Scotland: An Edited Corpus. Unpublished PhD thesis, Harvard University.

Hallén, Ywonne (1994) ‘The use of bone and antler at Foshigarry and Bac Mhic Connain, two Iron Age sites on North Uist, Western Isles’, PSAS 124, 189-231.

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