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Canadian Historic Sites: Occasional Papers in Archaeology and History No. 12

Lime Preparation at 18th-Century Louisbourg

by Charles S. Lindsay

The First Kiln

The Kiln

The site of the first kiln was prepared for construction by excavating a circular hole 23 ft. in diameter into the southeast side of the hill. A trench about 20 ft. long joined the hole on the southeast side to create a sunken passageway to the opening which served for both fueling the fire and drawing off the burnt lime.

The over-all diameter of the kiln was the same as the diameter of the hole since the wall was built against the side of the excavation. With a wall varying in thickness between 2.2 ft. and 3.5 ft., the average internal diameter was 18 ft. At its highest point the wall survived 10.75 ft. above its base (Fig. 11). On the southeast side the kiln wall thickened out into two short arms flanking the opening (Fig. 9).

The hole for the main body of the kiln was not dug to the same depth all the way round. At the entrance the base of the wall was at an elevation of 7.5 ft. above sea level, but on the north side it was at 9.0 ft. above sea level. Since there was undisturbed C horizon material against both the outer and inner faces of the lower courses of the wall, a foundation trench must have been dug around the circumference of the hole

A continuous bond between the footings of the retaining walls of the sunken passageway and the footings of the short arms flanking the opening, which in turn were bonded into the kiln wall, showed that the footings for the entire structure must have been laid at the same time. A gap was left in the footings of the kiln wall for the brick sill of the opening, which was laid directly on the exposed C horizon surface. This sill was laid with its inner edge following the curve of the inner face of the kiln wall; it was 2.8 ft. wide and extended through the thickness of the kiln wall.

With the foundations laid, the next construction stage was the erection of the kiln wall and arms. Since the wall was built against the sides of the hole, the outside face was irregular up to the pre-kiln ground level, above which it was smoothly faced, as was the inner face from the floor up. The junction between the rough and smooth finished outer face always occurred where the outside stratification showed the original turf line beneath the banks meeting the kiln, and revealed a drop in the surface level of over 2 ft. between the north and south sides.

While the walls were going up, the brick jambs of the opening were being inserted. These were inset from the alignment of the arms by approximately one foot and were bonded at the back in a rubble-stone offset that was a vertical continuation of the lip of the footings. The effect of this arrangement was to narrow the opening from 4.8 ft. to 2.8 ft. Excavation revealed that the jambs had survived for only three courses, above which they had been demolished when the approach was widened during the construction of the second kiln.

Over the jambs there would have been an arch, probably of flat stones laid radially around the curve, and possibly with an iron support band similar to those in the arches of the second kiln. There was probably also an arch over the two arms, but nothing remained of either one. Their existence is suggested by similar arches on illustrations of other kilns (Fig. 1). Comparison with these same illustrations and the arches of the second kiln suggests that the arch over the arms was probably higher than that over the opening, to facilitate access to the latter.

The bonding material for the kiln wall was a red, burnt clay which, when excavated, had a crumbly, brick-like texture. This material was originally clay mixed with water, or puddled clay, which was a bonding agent recommended for those parts of lime-kilns that were exposed to direct heat, since mortar would not withstand high temperatures. Mortar was used, however, on the arms of the kiln, which were not exposed to the heat of the fire.

The floor of the kiln was revealed in only a few places because the presence of the second kiln on top of it prevented complete excavation. One section was uncovered near the centre of the floor at an elevation of about 9.5 ft. above sea level, and another was exposed between the walls of the first and second kilns on the north side at a similar elevation. In both cases the stratification consisted of a re-deposited C horizon layer beneath the stone floor of the second kiln, below which was a thin layer of lime overlying the floor which consisted of the orange-red burnt surface of the undisturbed C horizon. The floor, therefore, was the bottom of the hole excavated for the kiln and the fire was laid directly on it.

The same stratification was present near the southeast opening, with the burnt surface at an elevation of about 8.2 ft. above sea level indicating a slope downward toward the opening. But the floor did not meet the opening at sill level. Instead it continued over the brick sill about 0.5 ft. above it and at the same level as the top of the surviving jambs (Fig 12). As it approached the opening, the stratification was the same as elsewhere, but over the sill itself the material underlying the floor was a re-deposited, stone-free C horizon layer. The sill level had been raised by depositing a 0.5 ft. thick layer of cleaned (i.e., stone-free) parent material over it; a step which must have been completed before the kiln was ever used since the burnt floor was on top of this layer of fill. This method of construction would also explain why the bottom three courses of the jambs of the opening survived. If they had been exposed when the approach was being widened, they would certainly have been removed since they would have obstructed passage to the second kiln. Since they did survive they must have already been hidden by the fill placed on top of the sill.

12 Section along the southeast passageway. 1, mortar-destruction layer (1755); 2, red, stony, clay loam fill for floor of passageway of second kiln (redeposited C horizon); 3, lenses of lime (probably represent dropping when first kiln was being emptied); 4, coal and burnt organic matter (remnants of fuel used in first kiln); 5, burnt, orange/red surface of undisturbed C horizon (floor of first kiln); 6, stone less, red, clay loam fill over sill of first kiln (redeposited, cleaned C horizon); 7, mortar (construction surface of passageway to first kiln); 8, red, stony, hard-packed clay loam (undisturbed C horizon). (click on image for a PDF version)

Presumably the sill was filled over to rectify a mistake made in the original layout of the opening. While this sort of explanation is obviously open to abuse as a catch-all for misunderstood or misinterpreted features, there is some evidence in this case to support it. First, since the brick sill was laid at the same level as the base of the kiln wall and since the base of the kiln wall was laid in the bottom of a foundation trench, then the sill would also be in the bottom of this trench. Second, the existence of this trench across the opening could be observed by the presence of undisturbed C horizon material immediately inside the opening at a level higher than the brick sill. The result was that the sill was about 0.5 ft. below the floor level of the kiln. It is easy to see that this could be overlooked when the foundation trench was being dug and the footings of the walls were being laid, and equally evident that in that position the sill was not functional. To remedy the situation the simple solution of raising the sill level was used. It is highly unlikely that this was an intended building sequence since there is no logical reason for laying a brick sill with the intention of filling over it; therefore, an error in the original design seems to be the most plausible explanation.

The Passageway

The retaining walls of the passageway to the opening did not bond with the arms of the kiln, but it is unlikely that this has any chronological significance because both the walls and the arms were constructed on footings that were laid at the same time. It is more likely that the butt joint reflects the building sequence of first the foundations, then the kiln wall and arms, then the retaining walls, rather than a later modification. The retaining walls were only one course thick along the sides of the passageway trench, but once pre-kiln ground level was reached they thickened out to 2 ft. and rested partly on the wall below and partly on the A horizon behind, the old turf line having been removed.

The floor of the passageway to the first kiln was originally dug to the level of the brick sill, and excavation revealed a layer of mortar, representing the construction surface, resting on the unweathered, undisturbed C horizon. However, when the sill of the opening was raised, the floor level immediately outside was raised as well, gradually sloping down to the original floor level further along the passage. Thus at the outer end of the passage there was a black layer of crushed charcoal lying directly on the mortar of the construction surface, but at the opening there was a layer of redeposited C horizon between the construction surface and the black layer. This black layer presumably represented the remnants of the wood fuel scraped out of the kiln after firing. On top of the black layer there was a number of lenses of lime which were probably dropped and spilled when the kiln was being emptied. It is curious that only one layer of burnt matter and one layer of lime were present, since one would expect a gradual build-up of alternate layers with each firing. Perhaps the kiln only survived one firing, during which the vitrification of the walls put it out of action.

A wood-lined drain 0.6 ft. wide and 0.5 ft. deep led from the outer end of the passageway in an easterly direction toward the sea (Fig. 13). The floor of the passageway near the drain was slightly lower than elsewhere and was probably dug this way deliberately to provide a run-off for the surface water that collected in the passageway, the floor of which is now well below the summer water table.


Around the kiln, banks of earth were thrown up against the wall to increase stability and reduce heat loss. Unfortunately it was not possible to determine either the vertical or horizontal extent of these banks in their original form because the excavation of the passageways for the second kiln added material to the banks and the destruction of the kiln during the second siege partly leveled them. The remains of the banks show that they were built from the material thrown up from the excavation of the hole for the site of the kiln, and that they extended completely around the kiln and along the arms and sides of the entrance passageway. Along the passageway the function of the banks was to provide more protection against direct winds into the kiln rather than to add stability to the walls.

The height of the kiln could not be determined from the remains. Historical sources and illustrations of similar kilns, however, indicate that it should be at least as high as its internal diameter, which in this case is 18 ft. Since the bottom 7 ft. or so were below ground level, 11 ft. or more would be left free-standing before the bank was constructed against it. The view of the lime-kiln at the Royal Battery (Fig. 23, c) shows approximately 6 ft. visible above the surface. If a similar amount were exposed on this kiln, the banks would have been 5 ft. high on the north side and higher on the south where the ground surface was lower. Since the natural angle of repose of the material used in the bank is 35°, the lateral extent of the banks would have varied between 7 ft. and 9 ft. This would not be excessive, but as the bank could nowhere be traced this far from the wall without either vanishing into the present topsoil as the old surface level rose on the north side, or being altered by the excavation of the passageways to the second kiln, it could not be proved archaeologically.

Some indication of the height can be obtained from the state of the kiln when it was excavated. The interior of the second kiln was completely filled with rubble thrown in when the kiln was being demolished during the second siege. This rubble, if used in building up the walls on top of the surviving remains, would result in a much higher kiln. In summary, the evidence concerning the height of the kiln is not conclusive, but favours a much greater height than the surviving walls alone would indicate.

13 Wooden drain leading from the southeast passageway. The dry stone wall across the end of the passageway can be seen running over the drain.


The excavated remains of the kiln cannot positively determine whether it was used as a flare or running kiln, since the distinction is based on the arrangement of the limestone and fuel. There is, however, some indirect evidence that suggests that it was fired as a flare kiln. Illustrations of 18th-century kilns that have a shape similar to this one are shown being fired as flare kilns (Fig. 3, d); that is, the fire is set on the floor of the kiln and the limestone is laid over and around it. The evidence of heavy burning on the floor of the excavated kiln also suggests that it was fired this way. If so, the process would be the same as that described for flare kilns.

The wall of the kiln was covered on the inside with a dark green glaze, and the fire-reddened stones were badly cracked on the inside face—the product of vitrification due to overheating of the fieldstone used in the wall. To the left of the entrance near the base of the wall inside the kiln there was a deposit of unburnt and partly burnt blocks of limestone with a thin coating of burnt lime over the top. Where the wall was protected by this layer it was not vitrified, indicating that the vitrification took place at only one firing during which the kiln became overheated rather than gradually building up over a series of firings. This limestone must have been part of the last load that the kiln fired, since it is difficult to believe that it would have been left in the kiln from an earlier load.

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