Burning Limestone on Mull

Our adventure began with a windy and wet visit to the Scotland Working Party on the Isle of Lismore. Despite the weather, we were inspired by a week of burning limestone in homemade kilns of concrete blocks and mud. With renewed enthusiasm, we set off for the Isle of Mull. Logic suggested a quick ferry journey, as we could see Mull in the distance. However, logic did not prevail. We had to travel back to Oban on the mainland to catch a later ferry. Fortunately, the ferry gods were on our side, and we managed to sneak onto an earlier boat, setting sail for the beautiful island.

Upon arrival, we were greeted by clear skies and a stunning sunset over the mountains—a stark contrast to the previous week. We were fortunate to stay at Duart Castle, a 13th-century family home of the Macleans, a clan renowned for their fighting prowess and heraldry. The castle sits on a headland surrounded by water on three sides, offering 360-degree mountain views—one of the best views I've ever seen.

Inspired by our previous lime burns, we spoke with our host, Andy Bradly, who mentioned rumours of elusive limestone on the Isle of Mull. Unlike Lismore, which is predominantly limestone, Mull had very little to offer. Rising to the challenge, we set out to find some limestone. A new road widening scheme had uncovered some light stone that seemed promising. Armed with a bottle of vinegar, we set off to see if we could get a fizz of calcium carbonate. Unfortunately, the fizz eluded us, and we returned to camp slightly disappointed. The Mull limestone was not going to give itself up that easily!

Determined not to be defeated, I accessed the university library to get the geology maps for the island. The maps showed two very small seams of limestone outcrops. Acid in hand, we set off again. Stop one yielded a fizz, but it turned out to be an anomaly. Stop two and stop three were both disappointments. Running out of time, we had to return to camp for a scheduled castle tour.

Unwilling to give up, I corralled Dan into the car one last time. We drove to the furthest reach on the map, an outcrop reaching into the sea. The road got narrower and narrower, ending in the middle of nowhere. On the roadside, we found remnants of white stone. Excitement turned to disappointment when we realised it was just an old bag of hydrated lime.

We set off on foot, crossing a cattle grid and trudging through marshland, fighting our way through long ferns. We faced our first obstacle: a narrow stream and bog. We crossed it modestly and continued through the undergrowth. Another successful stream crossing brought us to a ravine, the exact spot marked for limestone on the map. It was steep, and with no ambulances nearby, I quickly assessed the risks. Dan led the way, shuffling down the face first. Once he was safe, I followed. We got out the acid, sprayed it on the rocks, and it fizzed like sherbet dib dabs. We had found it—the elusive limestone of Mull!

We pulled out the hammer, broke off chunks of the seam, and loaded up our plastic bucket. Perhaps overly exuberant, we collected about 25kgs of stone. Scrambling back up the ravine, we crossed the first river and approached the second crossing and bog. Just as Dan was about to jump, I heard a scream. The earth had opened up, consuming Dan up to his thigh. Miraculously, he managed to extricate himself with his shoe intact. We made the last few hundred metres back to the car unscathed. A quick high five and a collapse on the bank in the hot sun marked our triumphant return to camp, feeling like heroes with our 20kgs of Mull's finest limestone.

Back at camp, we assessed the stone, which appeared crystalline and slightly metamorphic. We planned to burn it the next morning to see if we could make quick lime.

An early start saw us building a lime kiln out of a scrap BBQ grill and several concrete blocks. We sealed the blocks with a small amount of sand to retain the heat. Charging the kiln with our limestone and BBQ charcoal, we lit a fire underneath. Two thermocouples monitored the temperature. Once it reached around 900 degrees Celsius, we closed the bottom of the kiln and let it burn out. Now, we waited to see if we had successfully burned local lime, something not previously documented on the Isle of Mull.

As the sun set, we raked out the kiln. The limestone had turned a salmon pink colour and roughly halved its weight. Adding water to the stones, they fizzled up, cracked open, and slaked. We had done it—successfully created quick lime, which mixed into a beautiful, whipping cream-like consistency. We made a small batch of mortar and sent it off for analysis, eagerly awaiting the results, which would be fascinating to compare with the historic mortars in the castle.

This mini-adventure highlighted that everything we needed was within a stone's throw. It made us reassess the process of shipping lime from far afield when it might be available locally. While we can't all slake lime in our back gardens for practical and safety reasons, it does make us rethink the footprint of materials. Through careful choices and material selection, we can repair or build buildings that are not only in keeping with historic techniques but also sustainable and environmentally friendly.