Assessing the Long-term Success of Reigate Stone Conservation at Hampton Court Palace and the Tower of London

Figures & data

Table 1. Past Reigate stone conservation techniques. Works for which no reference is given are recorded in the archives of Historic Royal Palaces.

Date	Technique	Sites	Outcome
Early C19 (1820)	Roman cement	Bloody Tower gateway ceiling, ToL	Survey in 2002 identified on-going decay of stone beneath plaster.
1840s	Aluminium hydroxide	‘experiment made upon a cistern of Reigate stone’ (Anon 1844)	Unknown.
1882?	Wax coating	Widespread at HCP	Survey in 2008 linked rapid decay of Reigate stone plaque to impermeable wax coating. Similar decay patterns can be observed elsewhere, however there are also examples of well-preserved coatings.
Early C20	Barium hydroxide	Chapter House at Westminster Abbey (North 1930, 381)	Protective crusts soon delaminated.
1963	Calcium hydroxide (limewater) and various organic polymers	Salt Tower interior, ToL (Experiment including control areas)	Treatment had no significant effect (Clarke and Ashurst 1972, 43)
1984	Brethane (Alkoxysilane)	Upper Wakefield Tower, ToL	Surveys in 1999 and 2003 found that the treated pier, which had since been found to be heavily salt contaminated, was decaying more rapidly than nearby untreated masonry (Price 1993; Church 2003).
1988–1991	Lime method	ToL: Bell (1988), Wakefield (1990), Salt (1991), Cradle (1991) and Wardrobe Towers (1991)	On-going decay in many treated areas. Limewash coats detached rapidly in some areas, mainly attributed to high moisture content of masonry. Other coats still visible now. Casein, pigment and linseed oil variously mixed into limewash and lime mortar.
2001	HCT (solvent based)	Trials at HCP, ToL and Winchester Palace.	Some treated areas have resisted decay. Mineralogy and microclimate likely to be key factors.
2013	CaLoSil (Nanolime)	Trials at Westminster Abbey	Assessment of trials on-going.
2015	HCT (water based) and lime-mortar	Bell Tower exterior, ToL	On-going surface loss noticeable in exposed areas.

Figure 1. Cardinal Wolsey Plaque at HCP showing acceleration of decay rate on wax-treated Reigate stone surface.

Figure 2. Left: limewashed Reigate stone at Cradle Tower, ToL, shortly after treatment in 1990 and right: in 2018 showing good state of preservation.

Figure 3. Reigate stone at the Bell Tower, ToL, treated with HCT during the 2001 trials and 2015 conservation work. Greater decay is visible in untreated control (top right), than in stone treated during trial (second row); however, on-going, rapid decay is visible in powdering, darker stones (third row) compared to flaking, lighter stones (bottom row) treated in 2015.

Figure 4. HCT trial wall at ToL. Top: shortly after treatment in 2001 (dark surfaces due to residual moisture following treatment). Middle: individual stones in 2018 (each stone treated on right hand side only). Bottom: micrograph taken with Dinolite USB microscope, showing consolidation of treated surface.

Figure 5. DRM profiles of selected stones.

Table 2. Results of NDT (*due to deterioration of drill bit, values measured on separate stones are not calibrated).

	Surface hardness (HLD)			Absorption coefficient (g/m^2s^0.5)			Drilling resistance (N)*
	Untreated		Treated		Untreated	Treated	Untreated		Treated
	Mean	SD	Mean	SD			1–4.9 mm	5–25 mm	1–4.9 mm	5–25 mm
HCT.1	477	26	451	48	921	-	18.6	20.3	18.1	19.4
HCT.2	465	30	446	55	1021	620	7.4	9.4	7.4	7.2
HCT.3	451	51	426	62	1499	663	10.2	17.1	15.6	17.7
HCT.4	516	111	447	73	43	773	49.6	28.6	36.3	28.7
HCT.5	440	20	411	39	1496	1130	16	16.9	14.5	15.1
HCT.6	442	63	528	39	156	223	7	9	9.6	8.8
HCT.7	362	71	520	46	482	904	43.1	42.8	36.7	43.9
HCT.8	464	35	419	25	673	2179	23.3	26.9	19.8	17.3
Wax	345	47	422	57	526	91	-	-	12.7	7.4
Lime	-	-	314	83	-	1170	22.5	25.3	3.3	25.2

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North, F. J. 1930. Limestones: Their Origins, Distribution, and Uses. London: T. Murby and Co.

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Clarke, B. L., and J. Ashurst. 1972. Stone Preservation Experiments. Watford: Building Research Establishment.

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Price, C. A. 1993. “Preventive Conservation of Salt-Contaminated Masonry in the Wakefield Tower, HM Tower of London.” Institute of Archaeology Bulletin 30: 121–133.

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Church, L. 2003. “An Investigation into the Deterioration of Reigate Stone at the Wakefield Tower, HM Tower of London.” MA dissertation, Institute of Archaeology, University College London.

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