City of London Cemetery & Crematorium Crest

Nanolime in Modern Conservation

The City’s Crest, situated high above the main entrance to the City of London Cemetery and Crematorium was in an extremely perilous state. The severely deteriorated condition of the original Caen stone surfaces along with a multitude of different repair methods and materials, had created a highly complex and delicate substrate.

The consolidation properties and performance of nanolime were perfectly suited as a targeted and localised treatment for the decayed stone. This allowed the conservators to establish a solid base that could take further treatment and repair processes, without undue loss of original fabric. It also provided an ideal opportunity to further trial the material under site conditions.

Nanolime is a dispersion of calcium hydroxide in an alchoholic medium that can be delivered into a substrate. The alcohol evaporates, leaving the calcium hydroxide within the pore network of the stone. Once carbonated it becomes calcium carbonate, which partially fills the pores and bonds to both the host stone and one another to form a strong matrix. It significantly improves the mechanical properties of the decayed stone, such as the compressive strength and abrasion resistance and reduces the rate of deterioration, whilst maintaining the thermal and moisture expansion characteristics of the surrounding untreated stonework. It also has the benefit of allowing for any future retreatment or consolidation should it be required.

A series of workshop and site trials were undertaken using CaLoSil product range of nanolime. The aim was to ascertain the extent of penetration within the stone, degree of carbonation and the cohesion of the new calcite matrix and its bond with the host substrate. The best results were achieved using a combination CaLoSil E5, containing five grams of calcium hydroxide per litre of ethanol, and CaLoSil E25, which has twenty-five grams per litre. The E5 achieved greater penetration due to the lower concentration of lime particles allowing it to travel further into the matrix of the stone. The greater concentration of lime in the E25 can lead to clogging of the pores within the stone, causing the calcium carbonate to deposit closer to the surface and result in white blooming. It was found that five applications of E5, followed by three of E25, achieved the best penetration, with good carbonation and minimal surface bloom. The stone to be treated was carefully defrassed and all biological growth removed. The CaLoSil was applied using a brush and small syringe, which allowed for a more accurate and targeted treatment. The area was covered between applications to prevent premature evaporation of the ethanol and aid deeper penetration. The treated area was always pre-wetted and lightly sprayed with water throughout the process to maintain appropriate conditions.

The consolidation created a sound and rationalised surface to continue with the repair of the crest. Any surrounding stonework that was beyond salvage, such as the shield, medallion, Muscovy hat and motto were recarved in Caen Firm B Bed and previous cemetitious mortar repairs were replaced in a porous and permeable lime putty mix. Repeated ornament, such as the crockets and finials to the canopied niches, were reinstated with cast replicas. Silicone rubber moulds were made of the undamaged originals and then cast in a lime, prompt and aggregate mix. The crest was coated with a colour matched casein-bound limewash to unify the appearance and provide a further protective surface.