A typical procedure for applying Putty Lime Mortar repairs
Prepare the Mortar mix and allow some time for it to mature, whilst this is happening, we can prepare a sound, well keyed surface ready to receive the repair. Then flush loose dust from the surface and wet the stone several times with limewater. (Limewater is a saturated solution of lime, or calcium hydroxide)
Having previously mixed the repair mortar, remix the now matured repair mortar to a plastic state, and add 5% to 10% pozzolanic additive if required (Natural Pozzolans are usually materials of volcanic origin or sedimentary rocks of a suitable composition and are named after the town of Pozzuoli in Italy where the outpouring of several volcanoes formed beds of the material).
Now, before we apply the mortar, we must prepare a slurry of diluted mortar that can be painted onto the area to be repaired; when it starts to turn matt (as the water is absorbed into the stone) it is ready for the application of the mortar. If found necessary, prior to this wetting, wire or ceramic armatures may be affixed to the substrate to aid in providing further anchorage and shape to the mortar.
A Putty Lime Mortar Repair being applied, note the protective gloves and clothing
Remember, Lime burns, and so to apply the mortar by hand suitable protection such as wearing rubber gloves is necessary. The mortar can also be applied by a wooden or metal spatula to a maximum thickness of 12 to 15 mm (still remembering your protective gear). For deep cavities where mortar would take too long to “go-off”, you can use broken pieces of pre-wetted stone that could be pressed into the mortar, or a coarser aggregate used to take up the volume of the voids.
Return to compress the mortar after it has stiffened slightly, this is typically five to ten minutes after its application.
A second coat can then be applied when the first is leather hard, this is typically twelve to twenty-four hours after the first application. The surface of the first coat should be keyed before a second coat is then applied. We can build up and compress further layers as required, using a mortar slurry wash between each coat of the mortar.
The surface of the mortar can then be textured when leather hard by scraping back or compressing the mortar with a bristle brush. If required, an open granular texture can be obtained by pressing limestone aggregate into the soft surface of the mortar, with a damp brush or a shaped piece of stone. A smooth surface can be obtained by using appropriately sized and shaped wooden floats.
Finished repairs moulded to the shape of the original stonework, in time, these repairs will blend into the colour of the base stone and will help preserve the integrity of the structure for many years to come.
Finally, protect the mortar repair after the initial compression by using damp cotton wool, hessian or cloth. We must ensure the mortar remains damp for at least three to four days. Periodic re-wetting of the mortar during the first few weeks after the mortar’s application, is thought to aid in its carbonation.
The above information was gleaned from:
Aura Conservation Ltd’s work on St Georges Hall in Liverpool, with thanks to our Contract Manager Peter Jamieson for the Photographs
Stone Conservation, Principles and Practice, edited by Alison Henry (Donhead Publishing 2006)
Hydraulic Lime Mortar for Stone Brick and Block Masonry Various Authors (Donhead Publishing 2003)
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