This project was commissioned by Maura O’Gara-O’Riordan of the O’Gara Clan, with alter studies of the mortars funded by the Royal Irish Academy, including studies of dating the lime mortar (as opposed to charcoal or other material within a mortar). The purpose of the study was to analyse the mortars used in the construction of Moygara Castle, County Sligo (RMP No. SL044-052), and to identify and extract material suitable for radiocarbon dating.  The work was undertaken with the permission of the landowner, Mr. Frank O’Neill. Samples were taken under license from the Monuments Section, Department of Environment, Heritage and Local Government, and the National Museum of Ireland. The results of the radiocarbon dating are:

  •  Moygara A (Beta 252783): BP Date: 680 +/- 40 BP   Cal. Date (2 Sigma): Cal AD 1270 to 1320 (Cal BP 680 to 630)
  •  Moygara 3 (Beta 250444): BP Date: 640 +/- 40 BP   Cal. Date (2 sigma): Cal 1280 to 1400 (Cal BP 670 to 550) Bawn Wall
  • Moygara B (Beta 252784): BP Date: 340 +/- 40 BP    Cal. Date (2 sigma): Cal AD 1450 to 1650 (Cal BP 500 to 300)
  • Moygara 2 (Beta 250443): BP Date: 450 +/- 40 BP  Cal. Date (2 sigma): Cal AD 1420 to 1480(Cal BP 540 to 470)
  • Moygara 4 (Beta 250445): BP Date: 490 +/- 40 BP    Cal. Date (2 sigma): Cal AD 1400 to 1450 (Cal BP 550 to 500)

The radiocarbon results raised three key issues: the ‘Accuracy’ of Radiocarbon Dating of Lime Mortars; an overlap in dates obtained from the rectangular platform at Moygara Castle between 1280 and 1390 AD; and an an overlap between dates obtained from the gatehouse and bawn wall between 1420 and 1450 AD.

The ‘Accuracy’ of Radiocarbon Dating of Lime Mortars The radiocarbon dates obtained from Moygara Castle are not exact, and a number of factors must be considered in comparing the relationship of the dates provided by the AMS radiocarbon results and the probable construction date of the bawn, gatehouse and the rectangular platform.

The first factor is the conversion (calibration) of radiocarbon age determinations to convert BP (before present) results to calendar years. This has been carried out by Beta Analytic using the Pretoria Calibration Procedure. This should be understood as the best possible interpretation of the radiocarbon results at this point in time.

Old Wood: The second factor is the ‘old wood’ or ‘old charcoal’ effect – ie the age of the timber used in the manufacture of lime. Neither lime binder or charcoal radiocarbon dating provides an exact date for construction of a building. Dating of the lime mortar provides the date of carbonation of the lime (c. 6-24 months after the mortar has been laid in the masonry), while dating of timber and charcoal elements provides the age of the timber. The inclusion of charcoal within the sample ensures the samples cannot post-date the construction of the wall. Further, all studies of historic lime mortars to date agree that charcoal is a by-product of the burning of limestone to make ‘quicklime’, and is not an intentional aggregate. Therefore, all the charcoal was burned at the time of manufacture of the building mortar.  Therefore, the ‘old charcoal’ problem is unlikely to provide a cause for error in the results of the dating. The charcoal may be ‘old’ either because it came from the older rings of a large tree of considerable age, or it could have come from a tree that had died, but was only burned much later to obtain charcoal. It is not expected that dead trees would survive very long after dying in the Irish environment, except in specialized waterlogged conditions which would render the material unsuitable for burning. However, it is possible that a very old tree was felled for fuel for burning. The old wood problem is possible, but unlikely as traditional lime-burning practice favoured the use of small branches. Further, samples were obtained from very different parts of the castle structure. Given the massive quantity of lime mortar required for construction, it is unlikely that the same error would be obtained at all sites. Consequently, there is a reasonable probability that standard, traditional building practice was used, and small branches, or low chronological age were used for burning limetone, and that the resulting charcoal fragments should have a similarly young age.

The relative accuracy of the results of radiocarbon dating of Irish lime mortars can be compared with a previous study  of radiocarbon dating of charcoal contained of mortars from Cormac’s Chapel, County Tipperary. by R. Berger who checked the accuracy of his radiocarbon analyses against a historical reference recording that the building of a romanesque stone church by Cormac MacCarthy on the Rock of Cashel (begun in 1127 and consecrated in 1134):

 Previously Published  Radiocarbon Results – Extract from Berger, R. (1995) “Radiocarbon dating of early medieval Irish monuments”, Proceedings of the Royal Irish AcademyVol. 95C, Pp.159-174: Radiocarbon date: 900 +/- 40 BP (UCLA-2582).  Calibrated date:AD 1020-1260 Location of sample:  Internally from croft above the nave on the south side, 1.27 m from the western door jamb and 2.74 m above floor level”.Radiocarbon date:925 +/- 35 BP (UCLA-2584). Calibrated date: AD 1020-1210. Location of sample: Internally from croft above the chancel on the south wall. 0.42 m from the south-east corner and 1.32 m above floor level.

In this previously published radiocarbon dating study of lime mortars, the historical date for the construction of Cormac’s Chapel sits reasonably centered in the date range obtained from radiocarbon analyses

The Rectangular Platform at Moygara Castle: The rectangular platform, as it currently appears, is difficult to interpret as it offers little diagnostic architectural information. The platform has been alternately considered as the remain of a tower house, a hall-house or an artillery platform. The dimensions of the structure could be a substantial tower house or hall-house. However, it is unlikely to be an artillery platform. The position is not the best commanding position over the landscape and is overlooked from the south (i.e. the position would have been untenable under artillery assault from this direction), and the field of fire would have been severely restricted by the bawn towers.

The radiocarbon dates obtained from the rectangular platform provided an overlap in results between AD 1280 and AD 1390. While the potential for ‘old wood’ and ‘old charcoal’ must be borne in mind, this early date raises the interesting prospect that the rectangular platform may not be a tower house, but instead may be an earlier Hall-house or Hall-keep. The dimensions of the rectangular platform, absence of a visible ground floor entrance, and knowledge of other hall-houses in the region (Sweetman, D. (1999) The medieval castles of Ireland. Cork. The Collins Press, P.98) suggest that it is possible that the rectangular platform is a potential candidate for this medieval castle type. David Sweetman distinguished between Hall-houses and Hall-keeps.  The main difference between the hall-keep and the hall-house is that the former is only one element of a larger castle complex whereas the latter appears to be an isolated structure. Sweetman ibid. and others have identified c.35 hall-houses (though other authors disagree with this modern classification of a medieval fortification type), with Barry (2006) noting: The earliest group mostly clustered in Mayo, Galway, Limerick and north Tipperary has generally been dated to the early thirteenth century ( Barry, T. (2006) “Harold Leask’s ‘single towers’: Irish tower houses as part of larger settlement complexes”, Château Gaillard, Caen. Publications du Crahm. Pp. 27-33). There is consequently great potential for consideration of the relationship between the rectangular platform and the results of recent geophysical work by Kevin Barton within the context of the overall chronology of the site – and consideration of the date and categorisation of the type of fortification the rectangular platform. The radiocarbon results therefore suggest that further work is required to consider the nature of the rectangular platform at Moygara Castle.

The Gatehouse and the Bawn Wall: The gatehouse and the core of the bawn wall show overlaps in radiocarbon ages between AD 1420 and AD 1450. This raises a number of interesting questions about the nature of the bawn itself, and the relationship between the bawn and the gatehouse. The early date provided by the radiocarbon results is inconsistent with current knowledge of medieval fortifications. Consequently, either all three radiocarbon dates are ‘wrong’, our knowledge of the chronology of medieval fortifications is ‘wrong’, or the results are indicating that the bawn structure is much more complex than it first appears. At cursory inspection, the gun ports and corner towers of the bawn wall would suggest a late 16th or early 17th century date, and while artillery are firearms are known in 15th century Ireland, the bawn at Moygara Castle would sit uncomfortably in our current (imperfect) knowledge of medieval fortified structures in Ireland. It must therefore be considered whether the bawn wall and towers represent a single phase of construction (i.e. they were all built at the same time), or whether the bawn is a multi-period construction which was altered/extended or otherwise changed over time.

Consideration of the gatehouse building fabric suggests that building activity at the bawn is not a single-phase event. The gatehouse is not ‘tied in’ to the fabric of the bawn wall (a factor which would normally imply it is ‘later’ than the adjoining structure), though this location appears to be the sole entrance into the castle complex. The results of the radiocarbon dating suggests that the bawn and gatehouse are much closer in construction date than a study of the building fabric would suggest. This clearly raises a number of fundamental questions about how we understand the bawn, the gatehouse and the potential for alteration of Moygara Castle over time.

post-script: A subsequent project funded by the Royal Irish Academy examined the potential for radiocarbon dating of the lime mortars (as opposed to radiocarbon dating of organic additives or contaminants within the mortar).

Advertisements