The study of the Characteristics of Clay in Four Historical Monuments in Kashan

Background and objectives: Clay is one of the main earth materials used in architectural works from the past, with Iran considered as having one of the oldest histories of using earth materials based on climatic criteria. Over time, the use of these materials fell into neglect despite the county’s hot, arid climate and clay being a cheap, recyclable and highly accessible material. The importance of knowing these materials and analysing them during history is, therefore,particularly high. In addition, the sustainability and durability of some historic mud-brick buildings make it necessary to analyse and examine their materials in order to take advantage of the valuable experiences of the past and reveal their underlying techniques. Using clay and providing new solutions, therefore, is arguably pointless without examining and analysing the past. This is why we need to examine characteristic features of their construction in different historical periods to devise new solutions for their strengthening. With the aim of knowing the quantitative and geometric aspects of clay used in Iran’s historical buildings, this research has investigated these materials in historical buildings from different periods in Kashan, a rich example of the country’s historical textures. This study concernstheir appearance, chemical compounds, physical characteristics and techniques used in them, through library and field studies and various experiments and analyses of them. The results of the research can help us better know the materials and structurally analyse buildingsin order to identify and strengthen clay structures.
 
Methods: The research method is quantitative and based on field studies and laboratory experiments. The tests are divided into three categories. The first, basic tests, include density, specific gravity, moisture content, granulation and hydrometry and Atterberg limits. The second category includes compressive and bending strength, and the third category is the chemical analysis by XRD method. These tests were performed based on ASTM as well as the National Standards of Iran (ISIRI).  Four clay samples were tested from Kashan historical buildings ranging from the Neolithic to the Qajar periods. The samples werenamed with the letter C (standing for’clay’) and numbered in the order of old to new.
 
Results  & Conclusion: According to the tests conducted, one of the physical characteristics that are effective in the strength and quality of the samples is the specific weight or density. In this test, sample C3 had the highest specific weight and sample C2 also had a high specific weight. Moisture content is the amount of moisture in the sample and the lower it is, the better it is for the sample. This is because water absorption in wetter seasons causes contraction and expansion and eventually cracks. According to the obtained results, sample C2 had the lowest and sample C3 had the highest percentage of moisture. According to the location of sample C3, originated from the Seljuk fence, with a green space and a fountain currently in its vicinity, more moisture has been absorbed by the sample.
In the granulation and hydrometric test, all the samples had suitable and non-uniform soil and the amount of sand in C3 and C2 samples was more than other samples. In sample C1, the percentage of gravel was much higher than other samples and due to a lot of gravel in the clay, it can be said that the grading of this sample was more inappropriate than other samples. Also, in the Atterberg test, sample C1 had inorganic clay with medium pasting properties, and samples C2 and C3 had inorganic clay with low pasting properties. While the C4 sample had inorganic silt with low compressibility, it is better to use clay to prepare clay, because it has more adhesion and the soil grains are better placed next to each other. Therefore, the soil in C4 was not found particularly suitable for making clay.
The results of compressive and bending strength tests on clay samples are relatively similar. Sample C3 had the highest compressive and bending strength, followed by sample C2. The lowest amount of mechanical resistance was for sample C1. The mechanical resistance of sample C4 was also low and the reason for this was the use of recycled materials and poor-quality soil. Considering this issue, it can be said that soil has a direct effect on the physical and mechanical characteristics of the clay. In fact, in samples C3 and C2, the distribution of grain size was more suitable and the percentage of sand was higher, and this issue had created a uniform and dense texture between the grains and a stronger connection between them. Samples C1 and C4 also had non-extensive granulation, and this had prevented the creation of a uniform texture and less compressibility of the clay mass. In other words, the grain size and type of soil had a direct effect on the structure and strength of clay made from it. In addition, the specific weight and moisture contentwere also related to the mechanical structure of clay. According to the examination of clay samples, in general, it can be said that with the increase in specific weight and decrease in moisture content, the mechanical properties of clay, including compressive strength and bending strength, increase. In addition, based on the results of chemical tests, there are quartz, calcite and albite phases in all samples, and the higher the albite content in the samples, the higher the mechanical resistance.