For centuries lime was the only product widely available for mortars and renders. Lime production supported a cottage industry of stone kilns that stretched from John O'Groats to Lands End. The mortar was relatively easy to manufacture provided there was a source of limestone or chalk, a kiln that could reach a temperature of 900C and a pit dug into the ground to form a slaking tank. With the revival of stone building at the start of the medieval period, the use of lime became universal for the bonding and surface dressing of masonry right across Europe. The supremacy of lime continued until the late 19th century when Portland cements appeared.
Portland cement was preferred by the construction industry because it was faster setting, much stronger, easier to handle and more forgiving of misuse. And for dense modern brickwork and surface renders, cement mortars have performed well. It is arguable however, whether cement mortar is the best material for permitting movement and settlement of masonry.
According to Lachlan McDonald, a partner with consulting engineer Ellis & Moore, cement- based repairs are totally unsuitable for the repair of old masonry and stonework and can in some cases actually accelerate the decay of the masonry.w
'In restoring old masonry and brickwork it is imperative that the chosen repair material maintains the integrity of the original construction,' says McDonald. 'Old brickwork and stonework tends to be soft and permeable, therefore they should not be bedded with strong mortars which will cause the masonry to crack if there is any movement.'
Lime mortar, he suggests, is slow to set and allows movement within the mortar joints, while maintaining its structural integrity. Dense, impermeable cement mortars tend to confine and concentrate moisture movement in porous stone and brickwork, which can lead to problems of frost attack and premature decay after restoration work is complete. Such problems can then be compounded by water trapped on the cement mortar joints, which stand proud of the eroded stonework.
Lime can absorb and even allow moisture evaporation from a building. 'Lime mortars are particularly important in buildings constructed of dense impervious stone,' adds McDonald. 'Lime mortar joints, tuck pointing or lime washes provide a route for the evaporation of moisture, which is not available through the dense stonework.'
However, lime can be difficult to work with if you have had no experience, McDonald confesses. 'There is more to it than mixing it with sand and water and then trowelling it on like cement mortar. Lime materials must be carefully prepared if they are to perform well.'
Commercial lime putty is available from specialist suppliers, although on some restoration projects experienced tradesman tend to make lime putty in slaking pits or holding tanks, while other work is progressed. A good lime putty is produced after slaking for about three months during which time its free water content reduces as calcium hydroxide compounds settle out and consolidate. If kept long enough in a slaking pit or tank, the lime putty solidifies.
It is perfectly usable in this solid condition, although it will have to be mechanically agitated or 'knocked up' to entrain air and return it to a highly plastic state. For small quantities it may take 15 minutes of knocking up to make the putty workable. For larger quantities a mechanical pan roller mill is used. No water must be added to the putty at this stage, as it will impair the workability of the mortar and the setting process.
When applied, whether as a render or mortar, the lime gains strength slowly through re-absorbing carbon dioxide. It will tend to dry out well before its full strength is reached and this is controlled by the addition of a well graded sand and damp curing.
English Heritage is currently conducting a four year research programme on lime mortars ostensibly for the repair of Hadrian's Wall. Practical guidance on the use of lime mortars can be obtained from the Scottish Limes Centre Trust, The Limes Forum and Historic Scotland.