The conservation and rebuilding of a 143 year-old whalebone arch in the gardens of Cardigan Castle, Wales.

Most arches in the British Isles made from the bones of whales utilise the two large lower jaw bones, as is the case with this large whalebone arch in the gardens of Cardigan Castle in Wales. Most often, these were lower jaw bones of a Greenland Right Whale (Balaena mysticetus), a baleen filter feeder that lives in Arctic and sub-Arctic waters and grows up to 20m long, weighing up to 75 tonnes. The jaw bones could be up to five metres long though up to a metre would sometimes be buried in the ground when upturned and made into arches. It was fashionable to erect such decorative whalebone arches in the 18th century but the practice continued into the early 20th century. For more information, see ‘Whales Bones of the British Isles’ by Nicholas Redman.

Condition of the material: This particular arch in Cardigan originally stood on another site nearby and was almost certainly taller but the base of each bone was sawn off at some point, maybe when they were moved to the Castle. The bottom sections of the bones often rot away. These bones have stood outside and been exposed to the weather for about 143 years or so (they were presented as a wedding gift in 1871!). Unfortunately in recent years only one bone remained complete and still standing, the other was in pieces on the ground. The complete bone was in remarkably good condition. Although it had some ancient graffiti carved into it and it looked a little green with algae, moss and lichen, it was still firmly embedded into the ground. The other bone, however, was not only in several pieces on the ground but some major elements were missing. The largest piece was quite badly rotted, as were the four smaller pieces. This side of the arch had its convex side facing south and although both bones have benefitted from being in the shade of the trees (reducing extremes of wetting and drying, especially as the trees are mostly evergreens), it has become clear to me that it is often the south-facing convex surface of these bones that suffer from weathering the most rapidly. Once the spongy inner parts of the bone become exposed the bone weathers away relatively quickly and the bone begins to lose its strength. Unfortunately, not only was the top metre or so of the damaged bone missing but also much of the lowermost section. The broken pieces were in very poor condition, as they had begun to rot away and became discoloured by lying on and within the soil. The bone may have fallen partly because of being weakened by weathering but also the base may have been split by root penetration from nearby bushes and/or trees. This often happens.
Above left: the ‘arch’ in 2012 when assessed - Note how green the arch is, and that only one side still stands. Above right: The cleaned and conserved arch, with one side completely rebuilt. Below left and right: pieces of the broken side on the ground.

Conservation work undertaken: The complete bone and all the pieces of the other side of the arch were taken away for cleaning and conservation, transported safely on foam mattresses in the back of a van. All the pieces were gently cleaned with soft brushes and a vacuum cleaner, and small dental picks and tweezers where necessary. Not scrubbing them with water but keeping them dry. All the dirt and soil was removed, as well as the moss, lichen and algae etc. They all look much cleaner. The hole in the base of the complete bone, the ‘marrow cavity’, was full of small roots which went up through the middle of the bone for about 3 feet and then exited out of some of the blood vessel holes.
Once clean, the bones were consolidated to strengthen them and slow down the rate of weathering – particularly of the inner, spongy, bone. The consolidant (a thin conservation resin that penetrates into the bone) used was polyvinyl butyral (trade name Mowital) in isopropanol. This was applied using pipettes and brushes at a few different viscosities (2 percent to 15 percent) depending on requirements (i.e. the state of the bone).

Above: Cleaning the bone, measuring it and making the reversed model.

Rebuilding the broken side: Because much of the broken side of the arch was missing, there were only two possible ways of rebuilding this side: either using all the broken pieces and filling-in the substantial gaps with a weatherproof medium, or to make a complete model of the broken side and mould and cast this to make a single weatherproof structure. The former method would have resulted in a complex and very weak structure with lots of joins and made of at least two different materials which would inevitably expand and shrink at different rates once exposed to the weather, and therefore would eventually fall apart. Also, many of the pieces of the bone had warped whilst lying on the damp soil. The second method, however, would result in a structure made out of one single material without lots of joins and therefore would be inherently much stronger and would last much longer.
Therefore the complete bone was placed on a long bench and a plaster of paris model was made to match it as closely as possible, obviously in a symmetrically opposite shape. Measurements were taken and the model was slowly built up. Some of the original bone was incorporated into the model, to start the shape off and to give it the correct texture. The plaster of the model was carved in to, to give it the right sort of texture throughout its length to match the original bone as much as possible.
When the reversed model was complete, a two part mould was made using an expensive museum grade silicone rubber to pick up the detail and because the mould was so large and heavy it would have been at risk of tearing if cheaper rubber was used (the rubber alone cost about a thousand pounds). The rigid supportive part of the mould was made from Jesmonite acrylic resin with fibreglass, with additional wooden splints applied as the mould was over 3.5 metres long and was at risk of flexing.
When the moulding was completed, the model was de-moulded, the original bone incorporated into it was cleaned of plaster, and a cast was made with the mould using weatherproof Jesmonite acrylic resin with woven fibre glass matting. A strong internal metal support was made on a forge to fit into the cast and this was secured with further applications of resin. The cast was painted with acrylic paints to match the cleaned original complete bone, and was sealed with a Jesmonite matt sealant to weatherproof it. If the real bone turns green over the years and the cast does not, algae can be encouraged to grow on the cast by applying a thin mixture of yoghurt and cow manure, or a mixture of yoghurt, buttermilk and water with or without the manure. Or the bone could be cleaned.

Re-erection of the arch: The complete bone and all the broken pieces plus the new cast and the mould were all transported back to Cardigan Castle on foam mattresses. Holes were dug an appropriate distance apart (measurements had been taken before the original bone was moved) and to the right depth. A wide protective plastic tube (the upper half of a water butt) was placed into the hole for the real bone, to help reduce the chances of root damage, before the bone was placed into position. It takes at least 3 people to safely carry the heavy bone, and to carry the cast which is a similar weight. The tough Jesmonite resin should not be so easily damaged by roots so a plastic tube was not used in this hole. Some time was taken to make sure that the bones came together at the right angle at their apex, and that they did not lean forwards or backwards too much. Then, soil was placed back into the holes around the bone and around the cast, and was securely tamped-down with a large sledgehammer. This tamping-down should be repeated a few times over the next month or so as the bones and the soil settle. The bones were secured at their apex by drilling through both bones and inserting a threaded bar, secured with washers and nuts at both ends. The bones should be visually checked and given a little push in each direction to see if they move once in a while. However, their bases are so heavy and they are planted so deeply in to the ground that they really should not shift or be of any danger to anyone at all, even if people really try their hardest to push them over. To get them to move more than a fraction you would need a spade, plenty of time and some determination.

Recommendations for future preventive conservation: It would be worth re-consolidating the real bone every couple of years with the same consolidant. It would also be worth applying more protective Jesmonite matt sealant to the cast every couple of years. The evergreen trees and shrubs to the south of the arch should be maintained so that they continue to protect the bones from direct sunlight, ameliorating the extremes of environmental conditions. However, it would be best if trees and shrubs with aggressive roots are not planted too close. All these measures should help to protect the arch and ensure it stays standing for much longer

Above left: The cleaned bone with the old carved graffiti more readable. Above right and below, the completed arch.

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