G R A D I N G   A N D   Q U A L I T Y    C O N T R O L

The Irish Blue Limestone is a natural material and, like all quarried stone, has structural characteristics that reflect its geological history since formation. The durability of the limestone has been established by its use over protracted periods in Ireland but it is essential that the quality of the stone used in applications reflects the environment to which it will be exposed.

The quality control and grading system developed by the Irish Limestone Producers is based on the identification of a number of structural features found within the stone. These may be directly related to the environmental conditions under which limestones form or reflect the post depositional history of the deposit. The limestone grade is essentially a guide to the aesthetics of the material but also permits tailoring of the stone to more extreme applications. Features which, in the experience of the producers, are known to cause problems to the long-term performance of the limestone are excluded prior to the grading. The factors considered during the quality control and grading are as follows.

Fossil Fragments
The presence of fossils in the Irish Blue Limestone is normal and will not normally detract from the performance of the stone. Crinoid fragments are always present in the 'Blue' limestone and often make up the bulk of the material. They produce the speckle on sawn or polished faces. In the 'Shell' limestone, the proportion of crinoid fragments is often reduced giving a much finer grained and darker coloured matrix within which many large white bivalve shells occur. Most of the quarries also have 'Black' beds. These are fine grained with relatively few crinoid fragments and often have a marked grey colour on sawn faces. They do, however, take a very dark even polish.

Calcite Veins   
The diagenesis of a limestone is accompanied by a volume reduction and the development of minor fracturing. Deposition of secondary calcite within the fractures gives rise to white veining that may be particularly evident in darker limestones. The veins frequently form parallel to the bedding or the jointing of the deposit. The very fine veins, up to one millimetre in width, are often referred to as 'threads'. The calcite infil is generally in crystalline continuity with the adjacent limestone and will not usually cause problems with the stone performance. Although disregarded in terms of the limestone grade, 'threads' showing evidence of water retention make the stone unsuitable for use as a building stone and are excluded prior to grading.

The wider veins are more problematic. Where the calcite veins show any evidence of water retention or incomplete crystallization, the limestone is not suitable for use as building stone and would be excluded prior to grading. The presence of large poorly linked crystals, fine black or red lines or borders within or parallel to the veins, or small voids, are often indicative of problems. The larger calcite veins are obvious against the darker mass of the limestone particularly in honed or polished finishes and the aesthetics of the veining is the paramount factor in the grading. Well-bonded calcite veins do not detract from the limestone performance but as an additional safeguard the producers recommend specification of higher grade limestone for very large thin cladding sheets or where normal panels may be vulnerable to repeated impact.

Bedding    
All limestones are deposited in water and there may be periodic breaks in the sedimentation processes due to slight changes in environmental conditions. These discontinuities, which are called beds, are typically flat even surfaces that can be traced for considerable distances across the quarry. The bedding provides evidence of the original orientation of the sequence and it is important that this is reflected in the positioning of building stone. The limestone producers will, during processing, automatically orientate new stone in the correct plane to suit the particular application. Some evidence of bedding may be detectable in the Irish Blue Limestone usually as slight changes in the texture or subtle alterations in the colour. This is a natural feature of the stone and cannot be excluded completely.   

Where the bedding is marked by the presence of argillaceous (clayey) material there is a risk of water retention or breakage. This limestone would not be suitable for use as a building stone and would be excluded prior to grading.

Cherts    
Minor variations in the depositional environment of the limestone can cause the precipitation of cryptocrystalline silica that will form chert nodules ('Flints'). These normally appear in the Irish limestones as slightly irregular rounded masses; up to 100 mm in diameter; which are tightly bonded into the limestone matrix. The dark colour makes identification difficult. The extreme hardness of the silica may occasionally cause problems during cutting. Chert nodules do not impair the performance or durability of the limestone and are normally disregarded during grading.

Stylolites
The increasing thickness of overlying sediment during the formation of limestone produces stresses that cause the calcite crystals to cement together. The stress may also lead to pressure dissolution with the calcite dissolving into the pore water and being carried away. Up to 40% of the limestone mass can be lost in this manner. In all the darker coloured limestones, such as Irish Blue Limestone, the carbon content that gives the distinctive colour and other insoluble matter will be left behind to form a stylolite. Stylolites are three dimensional with both large and small scale structures present and the apparent carbonaceous zone can vary considerably in thickness.

Although often parallel to the bedding, the stylolites can be distinguished by their much more irregular form. Where the stone is cut perpendicular to the stylolite, it will appear as a thin crenulated line. If the cut face is parallel to the stylolite, it will often show as irregular highly convoluted circular patterns.    

Experience with limestone indicates that provided the stylolite is well bonded it is unlikely to give rise to large scale surface failure. After a long service life in very exposed positions, there may be some enhanced erosion immediately adjacent to the stylolite but this is usually an aesthetic rather than a structural problem.    

Stylolites which are not well bonded within the limestone structure make the material unsuitable for use as a building stone. The quality control is normally undertaken as part of the cutting operations. Experience indicates that the suspect stylolites show a tendency to retain moisture and at this stage usually show up clearly against the dry adjacent surface. Wetting the dry surface with a moist cloth is not a reliable test method.  

Experience in continental Europe, with limestones similar to the Irish Blue Limestone, has indicated that in certain circumstances there is an increased risk of surface failures associated with the presence of stylolites. The likelihood of failure is apparently related to the size, the degree of cross bonding, including mechanical interlocking of the convoluted surfaces, and the orientation of the stylolite relative to the exposed face. Well-bonded stylolites, which are not visible on the surface of the stone when viewed from a distance of 3 metres, can be disregarded in terms of durability.   

Although similar problems have not been experienced with the Irish Blue Limestone, the producers have taken account of this potential risk in their Code of Practice when grading stone for export.   

Where the limestone is set on the bed, as in ashlar, visible stylolites within 20 millimetres of the upper and lower internal joints would exclude the stone for use in continental Europe. Unless the conditions are very severe (e.g. subjected to frequent hard frosts or salt spray etc.) this stone will normally give satisfactory performance in Ireland.

Limestone capping stones, sills or the top and bottom courses of ashlar should not have visible stylolites within 40 millimetres of the exposed surfaces for use in continental Europe. Except in very severe conditions, stone not complying with these requirements is unlikely to cause problems in Ireland.    

Where visible stylolites occur on the exposed faces of limestone cladding or slabs, the conditions that may make the stone unsuitable for use become quite complex. The presence of a well-bonded stylolites that traverse the face will not normally affect the durability of the limestone. Their average width, based on 10 measurements along the length will, however, be reflected in the limestone grade.    

Higher grade limestones should normally be specified for very large thin panels, more severe conditions or where the finished panel may be subject to repeated impacts. If the stylolite intersects the cut face in a manner that gives rise to a series of isolated pockets of limestone, there is a slightly increased risk of surface failure. 

Experience with the Blue Limestone in Ireland has shown that where the 'visible' stylolite position gives rise to numerous small concave structures which penetrate a few millimetres below the surface, there is a slight increase in the risk of surface spalling in exposed conditions. This will affect the aesthetic appearance of the stone rather than the mechanical strength. Where the stylolite penetrates more deeply into the stone, it is unlikely to give rise to surface failure in Ireland.   

The continental European limestones have been shown to develop surface spalling when larger stylolites give rise to concave structures that penetrate up to 50 millimetres below the surface. In view of this experience, the Irish Blue Limestone for export is subjected to rigorous quality control. Limestone panels or slabs less than 50 millimetres thick, with large stylolitic structures that fully penetrate the stone, are deemed unsuitable for export. Where the Irish Blue Limestone is supplied as fully finished items, all 'visible' stylolites on the exposed faces will be carefully assessed to exclude vulnerable concave structures. Certain mechanical finishes may increase the risk of failure on otherwise acceptable structures associated with stylolites.

Soft Patches
The presence of soft or unconsolidated material within the limestone usually arises when clay or silt is present in sufficient quantities to interfere with the crystalline growth that accompanies normal diagenesis. This is most likely to arise with 'Shell' type limestones where more clayey material is trapped within the shell and should not be confused with paler or more crystalline deposits that may also infil the shells.    

Leaching of calcite due to the percolation of water through the natural joints and fissures or pressure dissolution during the formation of stylolites can also give rise to softer more porous material. Soft, very porous or unconsolidated patches result in automatic rejection of the limestone for use as a building stone.

White Masses
Calcite, which is the dominant mineral in the Irish Limestone, may be occasionally found as pure white inclusions. Where the white masses are well bonded into the groundmass, they pose little threat to the durability of the stone but may detract from the appearance of the cut stone. The grading system of the limestone takes account of the size of individual white masses and the proportion of the surface formed by white masses.

Colour Variation
The colour of the Irish Blue Limestone is due to the presence of finely disseminated carbon and some colour variation is normal. This is often most apparent between slabs cut from different blocks but may also arise from variations in the environmental conditions within a single block. The producers will normally try to maintain uniformity of colour but it must be recognised that the material is a natural stone and some variation is unavoidable. The formation of a surface patina due to weathering of exposed faces will naturally attenuate the colour variations that may seem very apparent when the freshly cut stone is first placed.