analysis of the thin sections the following conclusions were drawn:
1. Difference in mortar composition between the bedding mortar and the pointing mortar is clear: pointing mortar has a higher binder content and finer fraction while the sand has almost no greenish (glauconite) grains that are found in the bedding mortar.
2. Nodules of other materials, probably cinder track are found in the mortar;
3. Adhesion between mortar and bricks is sometimes very good and sometimes poor; this could be due to a multitude of reasons, including the preparation (polishing) of the thin section;
4. It seems that between bedding and pointing mortar and in case of M8 between 2 layers of pointing mortar some "deposit" can be seen; this may be an indication that the pointing mortar has been applied later, which could be consistent with the use of another sand.
Masonry conservation of historic structures poses substantial dilemmas for architects, conservators, and craft workers seeking to incorporate new mortars with composition, performance, and characteristic properties compatible to those of original mortars.
In the late 20th century and early 21st century conservation philosophy embraces the incorporation of original masonry materials and techniques where possible. Properties of the mortar used for repair are critical when in contact with the original remaining mortar parts that have to be conserved. Since matching the composition is no guarantee that the two mortars will have compatible characteristics and future performance, it is essential to work toward a better understanding of original and repair mortar properties. Therefore it is crucial that the craftsman is involved in formulating a repair mortar, as he has to do the job with products or materials he may not be acquainted with.
Therefore, an important issue during the restoration of the Fort IV was the repointing experiment of the masonry walls as technical, historical and aesthetical arguments were in play. However it may show how important the intensive discussions with the craftsman before and during pointing were. In general we state that due to the lack of participation (bottom-up) within conservation, the craftsman is excluded (top-down) from the general thinking and decision making process. He's familiar with compatibility and reversibility concepts, yet he isn't acquainted with general conservation philosophies or interests.
As argued by K. Van Balen (Van Balen 2003) it is clear that a uni-directional communication would not lead to an authentic conservation intervention. In order to avoid poor craftsmanship due to un-addressed know-how, it's important that the different parties discuss from the beginning the final technical, but also the philosophical context of the conservation.
Architects could act as initiators of this debate in the conservation team, as the present-day craftsmanship training often focus on technical aspects and hardly includes more theoretical and philosophical questions. This early debate could help to assure the involvement of proper craftsmanship.
In that respect the experiment in Fort IV was unique as it tried to embrace the knowledge of a craftsman in an early stage of the conservation process. So, a dynamic interaction between pointer, experts and architects arose as the pointer was asked to give proposals (bottom-up) of possible mortar mixtures within the guidelines formulated by the architect and the guidelines resulting from the historical investigation and the laboratory tests. In order to prevent a top-down approach that obliges the pointer to work with a certain mortar composition, that he doesn't know or isn't familiar with, the strategy of trial and error was followed.
During the iterative process comments of the craftsman were always collected after each evaluation of a set of samples. Many comments were gathered during conversation when the site works were ongoing.
For the proper conservation of the decorated masonry, two types of pointing would be needed, a dark and a light mortar. Further flush and cut-to shape joints were requested.
After some preliminary discussions with the craftsman, a first series of three samples was proposed with the following compositions (in volume ratio):
- Samplel: 1 part trass lime (German ready-mix product with >55% trass) + 4 parts sand + V part "grey lime" (name and product proposed by the craftsman)
- Sample 2: 1 part trass lime + V part grey lime + 3 parts sand + 1 part yellow sand
- Sample 3: 1 part trass lime + 3 part sand + 1 part yellow sand + V part lime hydrate
According to the pointer the different samples had more or less equal workability. An initial trial of sample 1 with 1 part of grey lime in stead of a V part resulted in a fatty lime, and was therefore according to the pointer unsuitable, as the binder-to-sand ratio could have been too high.
After the evaluation of these samples a more greyish colour mortar was requested by the architect. Therefore a new set of samples was placed by the craftsman. Meanwhile the craftsman was shown a video that explained the use of lime in pointing mortars:
- Sample 4: 3 parts sand + 1 part trass lime
- Sample 5: 3 parts sand + 3 parts of Rhine Sand + 1 part trass lime + 1 part of grey lime
- Sample 6: 6 parts sand + 1 part trass lime + 1 part of cement (CEM II/A LL 32,5 R)
- Sample 7: 6 parts of Sand + 1 part of trass lime + 1 part of lime hydrate
The pointer indicated that he normally would have added some cement in sample 4 to create a bastard
mortar, which he preferentially applied in his projects as such mortars feel less "fatty". As the demand was to use mortars without cement he proposed the same composition by omitting cement. He confirmed that this sample was quite difficult to work with, possibly because the overall binder to sand ratio was quite low.
In his opinion Sample 5 was too coarse to create cut-to shape joints. Yet, the sample had good workability, but he was afraid to use another type of mortar for the cut-to shape joints as colour differences would have been possible.
The evaluation of the samples demanded for a last set that could help to determine the final choice:
- Sample 8: 3 parts of Rhine sand + 1 part of trass lime
- Sample 9: 5 parts of sand + 1 part of Rhine sand + 1 part of trass lime + 1part of lime hydrate
- Sample 10: 6 parts of Sand + 1 part of trass lime + 1 part of cement
Sample 8 was inspired by sample 4, yet with another type of sand in order to obtain a colour that would match better. Nevertheless the pointer had to admit that this sample was too coarse to apply in the often narrow joints. In order to make sample 5 less coarse he made some adjustments to obtain the mortar composition of sample 9. Although the workability improved, and it was easier to apply in the narrow joints, it was still too coarse to obtain a fair result.
Sample 10 finally illustrated the usual compositions with a binder to sand ratio of 1:3 and the use of cement and lime.
Eventually sample 2 and sample 6 were chosen for the light and dark mortar joints.
As in one of the buildings (the left "capponiere") the masonry showed a patchwork of light and dark coloured bricks, the choice for the appropriate mortars
was less straightforward. It was decided to use the light coloured mortar (nr. 2) for the complete façade. The craftsman would have preferred the dark coloured mortars, but he said this was just a matter of taste.
After the works were finished, the pointer admitted that he was initially quite reluctant to use the trass lime that was stipulated in the original job descriptions of 1859, as he wasn't familiar with the product. He even had problems to find the appropriate product as he only had found the first bags in a "garden shop" close to Antwerp. Therefore it was important that the pointer could "experiment" with the trass lime in order to obtain insight in the workability properties of the product, and that he could try several different mixtures.
Eventually he was quite surprised that the trass lime was easy to use and that a considerable strength was obtained after some days. He mentioned that he has been using trass lime since in other works after his experience in Mortsel. So, one can argue the importance of initiating craftsmen in 'forgotten' traditions, as the use of a certain material is important and that the reluctance is often more related to habits than to bad will.
The pointer applied in almost all the cases the so-called "velco" sand, a product that originates after washing the so-called Lommel sand. The product is somewhat coarser than the Lommel-sand and therefore it is more difficult to make cut-to shape joints. The craftsman was well aware of the problem, yet he was afraid that another composition would differ too much in colour from the approved samples.
The pointer experienced several problems when brushing the joints, because in a prior phase the facades were cleaned with a slightly abrasive sand-water mixture. In his opinion several bricks were damaged at the surface by this operation, the brushing of the joints polluted the bricks with mortar remains that were difficult to remove.
Some argue that many craftsmen are only interested in technical and aesthetical compatibility and have few questions regarding the final conservation philosophy or principles. Nevertheless, our craftsman tried to identify peculiar problems, that in his opinion could need another approach but were difficult to judge as his training never included these more philosophical issues. The most illustrative example that he showed was the cut-to shape repointing of an area with original brickwork. In this case the outer layer of the bricks was damaged. The original joints had fallen out and needed replacement. The craftsman struggled with a technical-historical dilemma: would it be sensible to create cut-to shape pointing as the outer layer of the bricks had disappeared completely? His final judgement was more practical by following the general rule that window arches were originally with cut-to-shape pointing.
It shows that the input from the craftsman, a bottom-up strategy, from the beginning of the project can be very useful to guide certain decisions.
Eventually the craftsman was quite satisfied with the overall pointing result and with the guidance and discussions during the project. Nevertheless he had to admit that he had lost a lot of time at the beginning when searching a correct pointing mixture. Further, he found it quite difficult at the start to understand the global framework of the different trials. He was very satisfied that he could work fast and effectively with the trass lime, a product he had never applied before.
It should be said that the craftsman had a very open mind towards this experiment and that therefore he kept on trying to improve his samples to obtain the desired result.
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