Bisya Area Site - Al Hajar Project: Conservation Report
Richard and Helena Jaeschke Nov - Dec 2011
The conservation work began on 28th Nov 2011 and finished on 17th Dec 2011.
Following from the report submitted to the Ministry for Heritage and Culture in 2010: Conservation Proposals for the Site of the Al-Hajar Project, two trial areas were chosen for stabilisation treatment, both in Building 4/7 on the Sallut plain.
There had been some damage to the site from the recent flood waters, which had entered the excavated areas on both the eastern and western sides. An area of the southern baulk, approximately 2.5 metres long by 40 cm wide had collapsed, along crack lines which had been evident during the investigation visit in March 2010. Dried sediments on the base of the excavated areas and on the upper surfaces of most of the stone blocks showed that a considerable depth of flood water had entered the site, depositing silts. A safety tape was laid out along the fragile baulks to warn people not to stand too close to the edge.
The two walls chosen for treatment were the wall of mixed large and small stones which runs west-east just south of the circular wall of the platform monument, and the most southerly exposed wall, made of very large blocks. Each wall was treated up to the edge of the exposed areas, where adjacent tumbles of stones and sediment have not yet been excavated.
In addition, four small test walls were constructed outside the fenced area of the monument, using stones which were lying piled nearby to the east of building 4/7. These were erected using the silt from the surface of the plain, mixed with water, as a rough mortar, to mimic the earth packing found in the walls. It is not yet clear whether mud mortar was used during any phases of construction in the monument itself; many of the stones appear to have cracked when further blocks were placed on top of them, which implies that they were lying unsupported on the uneven surface of the lower courses. Construction is much easier if a mud mortar is used, but requires a considerable amount of water.
Plants, which had grown extensively in the excavated areas of the site, were cleared by a team of 6 workmen from Bisya, provided by the Ministry for Heritage and Culture. They also constructed three of the test walls, copying the first wall which was built by the conservators.
Materials for the conservation work were gathered and taken to the site – washed sand and sarooj (6 bags of each, kindly provided by Ahmed Al-Tamimi and Ali Adawy from the stores at Bahla fort) and hydrated lime (2 bags purchased from Construction Materials & Industries at Al Amerat, near Muscat) as well as a water tank, filled by the local water station just outside Bisya.
An area of the wall approximately 1 metre wide by 1 metre high was treated by each conservator at a time. The loose, cracked silts on the upper edges of the blocks were cleared away using pointing tools and brushes and any remaining earth packing behind removed to a depth of about 7 – 10 centimetres, depending on the size of the blocks and the gap between them.
The area was then sprayed lightly with water and filled with a mortar made from sand, sarooj and hydrated lime. The proportions for the mortar were those recommended by the Dept of Civil Engineering at Sultan Qaboos University after testing[1]: 3 parts sand, 3 parts sarooj, 1 part hydrated lime and approximately 2 parts water, measured by volume. The sarooj was sifted to remove large pebbles and unbroken lumps and mixed with the dry sand and lime before the water was added. Enough water was added to provide a mixture which was soft enough to bind together when compressed, but not too wet as this would allow the material to slump too much and cause the finer particles to migrate to the surface.
The mortar was firmly packed into the joints of the stonework, using spatulas and small trowels. It was built up to within about 1 centimetre of the surface in small joints and areas around small stones, and about 2-3 centimetres of the surface in wider joints and areas between very large blocks. A considerable amount of judgement needs to be exercised when determining the best level to fill the mortar, according to the shape and arrangement of the stones in each part of the wall. Care was taken to make sure that as little mortar as possible was applied to the front surface of the stones. Any small spatters which fell on the surface of the stones were immediately removed and sponges dipped in water were used to wipe away any wet smears.
As the mortar was drying it was sprayed lightly with water at intervals. A tarpaulin was arranged over the work area to keep it shaded and cooler, allowing the mortar to set more slowly. This allows the mortar to achieve greater strength.The tarpaulin was held in place with ropes tied to the fence posts and weighted down with loose stones brought in from outside the fenced area. Each loose stone was marked “O” for “outside” with a waterproof, fade proof black marker pen.
The next day the mortar was stiff, but still soft enough for the surface to be cut back a few millimetres to a level surface, using flat-ended spatulas and a fine steel wire brush. This neatens the surface of the mortar and exposes the larger sand grains, providing a more natural appearance.
The work progressed from the bottom to the top of the wall, in approximately 1 metre squares. Adjacent areas were left untreated until the mortar was strong enough to be load-bearing and support the stones, then the remaining areas were treated. Each conservator could clear and fill approximately 1 square metre per day and the 15.28 square metres of the first wall were completed in less than 8 days. The mortar will continue to harden for several months as it absorbs carbon dioxide from the atmosphere.
The second wall, built from much larger blocks, with larger gaps between the stones, required a slight modification of the technique. Some of the joints were so wide and tall that the mortar would slump and sag if attempts were made to fill the entire area with one application. Instead, the joint was filled in stages, applying a layer of mortar approximately 8 – 10 centimetres deep and allowing it to harden for an hour before applying the next layer. This wall, approximately 13 square meters, was completed in less than 5 days.
Parts of broken blocks were correctly positioned using small wooden wedges to hold them in place until the mortar was applied and had hardened. Once the mortar was firm, the wedges were removed and the small holes filled with mortar.
A small piece of partially carbonised wood was found between two blocks at the eastern end of Wall 2 as the loose silts were being cleaned away. This was kept as a possible sample for Carbon 14 dating.
Materials used
The conservation of 28 square metres of wall used 5 bags of sand, 4.5 bags of sarooj and 1.66 bags of hydrated lime, mixed into approximately 42 batches of mortar. Approximately 189 litres of water were used in the mortar and 192 litres were used for spraying the wall to keep the mortar cool and moist while it was hardening.
The test walls were treated with different mixes of mortar: one was left with just the mud mortar fill as a reference. The mortars used in the other walls were:
Wall / Sarooj / Sand / Lime / Water2 / 3 / 3 / 1 / 2.25
3 / 2 / 3 / 1 / 2
4 / 4 / 3 / 1 / 2.5
The mortars will be examined next year to see how they have aged during the intervening months. If necessary, the mortar mix can then be adjusted for use in the treatment of further areas of the monuments.
A safety enclosure was designed by the conservators and made from steel by Kausar Ahmad at Healty Tools and Metals in Bahla. This will be used when taller or less stable sections of wall are being treated. The enclosure allows the conservators to work at the base of the wall and will protect them from possible falling fragments of masonry, allowing them time to scramble clear if any large blocks detach from the top of the walls.
Flood protection
The conservators had hoped to create two berms or low walls of earth and stone using material from the old spoil heaps near the site, to divert floodwaters away from the excavated area. The workmen created a short berm on the northwest side of the site, but the JCB did not arrive to construct the longer berm on the east side of the site. It is hoped that the Ministry of Heritage and Culture may undertake this work before the next rainy season.
The authors would like to thank the Directors of the Hajar Project, Jeffery and Jocelyn Orchard for enabling them to undertake this work, Dr Biubwa Ali Al-Sabry for granting permission for the work and Ahmed Al-Tamimi and Ali Adawy for their support with materials.
[1]Hago, A. W., Al-Rawas, A. and Al-Harthi, A.S. “Determination of Optimum Volume Proportions for Sarooj (Pozzolan) Lime Mixes” in Environmental and Engineering Geoscience Vol VI, No 2, May 2000 pp 171-176