Masonry Magazine October 1997 Page. 25
Union Station, Indianapolis, Indiana, built in 1888
years gaining strength as it coverts back to limestone within the wall.
Mortar Preparation & Mixing
Mortar made with lime putty and sand (referred to as "roughage" in Canada and "course stuff" in Europe) should be measured by volume. Very little, if any, additional water is usually needed to bring the lime mortar to a dry workable consistency. Sufficient water is already supplied within the putty itself. It is important to keep the water to a minimum for two reasons. First, a drier mortar is cleaner to work with and enables the material to be compacted tightly into the joints. Secondly, the mortar cures with no shrinkage cracks due to the evaporation of excess water. Sand should be proportioned first followed by the lime putty and mixed for five minutes or until all the sand is thoroughly coated with the lime putty. The final consistency should resemble that of brown sugar.
But mixing, in the familiar sense of turning over with a shovel, was not considered sufficient in ancient times, nor is it sufficient now, if the best possible performance is to be obtained from a lime putty mortar. The old practice of chopping, beating and ramming the mortar has largely been forgotten. Recent field work has confirmed that lime putty and sand rammed and beaten with a wooden mallet or ax handle, interspersed with chopping with a shovel, significantly improves workability and performance. The intensity of such an action increases the overall lime sand contact and removes any surplus water by the compaction of the ingredients. It may be advantageous for larger restoration projects to utilize a mortar pan mill for mixing. Recently introduced from England, this machine is extensively used throughout many parts of the world to mix historic lime putty mortars. Its design is centuries old and produces a superior mix not attainable with today's modern paddle and drum type mixers. The total volume of water necessary may vary from batch to batch, depending on weather conditions. Unlike mortar formulas that contain portland cement, lime putty sand mortars can be re-worked at the site over and over again cutting down on waste. They are not restricted to be used within 1 1/2 hours after initial mixing.
For larger projects the lime putty and sand can be combined together ahead of time and stored indefinitely, on, or off site. The need for sand piles on the job site can be eliminated. Lime putty and sand can be mixed again months later back into a workable plastic state with no
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