The canvas used as the support of painting does not refer to falx in the simple texture. Textiles made of hemp, silk, flax, synthesized fiber in the plain texture and sateen are called canvas, too. Up to the 19^th century, the hemp was made merely out of flax and hemp fibers. However, cotton gained popularity after the 19^th century.

Compared to flax and hemp, silk has been used less in western paintings. Small paintings on flax or silk cloths with fine and dense texture were preferred. Hemp was often used for the coarse canvas.

Sometimes, the secondhand cloth such as tablecloths were used for painting. The fabric and thickness of fibers, types of texture and its denseness affect the flat surface of the canvas. The texture possibly refers to the place where the artefact was painted or the period of its painting. In spite of reliable dating for the fiber by means of scientific methods, one cannot rely account on the texture for determining the origin or date of painting. In the early years of the 16^th century, most of the supports were made of cloth and they had simple and dense texture. Then, more coarse or tilted texture became popular particularly in Venetian paintings.

In the 17^th century, the coarse texture with some openings among the fibers gained popularity in Italy. This type of canvas was used in the 18^th and early 19^th centuries, too. As of the second half of the 18^th century, canvases with finer, denser and more consistent textures became so popular. In the 19^th century, the machine-made canvas produced by electric weaning machines gained prominence. This type of canvas is distinct from other ones due to its homogenous texture.

Primer

In murals, the primer refers to the layer placed on the surface of the support as a coating or covering. The main function of the primer is leveling the surface of the support and creating an appropriate surface for further layers. One of the most common primers in decorations related to Iranian architecture is the mud primer whose main composing element is the clay. Some other elements such as plant fibers, wool or hair of animals and sands are often added to improve its performance.

The oldest mud primer is the mud coating on the adobe walls of Teppe Gang Darre in Kermanshah that dates back to the 8^th millennium B.C.

Teppe Gooran in Kermanshah contains thatch coating on the adobe walls performed by mud grout. Some other cases have been found that date back to the 7^th and early 6^th millennia. One of them is Teppe Zaghe in Qazvin where one can see the thatch coating on the clay walls and adobe walls.

Characteristics of adobes of ‌Teppe Zaghe

The adobes of this mound are cube-shaped. They are stretched with some grooves and holes on. The holes are useful for ease of carrying the adobes and locking them together. Grooves are incorporated to increase the rate of grout getting inside and getting two series of adobes interconnected.

The best material used as the primer on Iranian works in various periods is the plaster primer. Some other materials are added to this primer to improve its quality. The plaster used as the primer should be processed quickly and it should be of the large grading.

Ground

The ground connects the support to the paint layer. In the oil painting, the ground sticks to the support and it preserves the paint layers on their places as the paint penetrates in these layers and binds them together. The ground makes the texture homogenous. As a result, a homogenous ground created which is in harmony with the texture and the paint. The ground can cover the texture of the support or fortify it. Additionally, the ground can make a separate and independent texture.

In the murals, the ground refers to a layer placed on top of the primer. The ground closes the big openings and creates a soft and smooth surface. Researches on Iranian works have revealed that plaster grounds have been used there and they have two features as follows:

1. Finer grading
2. Different processing (dead burnt plaster is processed)

In the spaces with high humidity such as the humid regions of Iran and/or baths, they make use of the lime primer. Due to the sensitivity of the plaster to the lime, it is used in the humid regions. The grinding of the lime in the ground is much finer than the primer.

In some cases, the mortar is used which is made of lime, ashes, sands, and plant fibers.

Unlike the northern and southern areas of Iran close to Caspian Sea and Persian Gulf, the plaster has been used in central regions of Iran due it its suitable features. The plaster has been used as an element in the architectural structures or as a coating for leveling the surface or as a plasterwork for decoration of some surfaces.

The dead burnt plaster reduces the hardening potentiality and when it turns into powder it is considered as amateur and possibly increases the resistance.

Another material used for these purposes is the hydraulic plaster which is made by adding 10% lime to the plaster or when the lime is baked. The prominent feature of the hydraulic plaster is its resistance against humidity. It is used for making floors. The particles are in the form of lively lime. No sooner does it absorb water, the mortar increases in size which leads to emergence of some cracks or blisters on the surface.

Hardening:

Hardening is a process thorough which some needle-shaped and column-shaped crystals are formed that form an intricate network. They contribute to the hardening of the plaster. The dead burnt plaster of these crystals are located in a further distance from each other.

Dead burnt plasters contain more water compared to the gypsum and they are processed. The dead burnt plaster needs 70% water. The huge amount of water results in porosity, lack of accumulation and lack of resistance.

The hardening processes of the plaster are isometric, that is, they release heat and after hardening the plaster expands in volume. The beginning time for the hardening should not be earlier than 2 to 4 minutes. The process is expedited within 10 minutes. Tragacanth is assed to the water that is supposed to be poured on the plaster. Milk and thorns could delay the time of hardening.

Natural adhesive of tannin is extracted for the Eremurus roots whose roots are thick and glazed. The roots are dried up and then grinded. Then, it is turned into powder and used as the adhesive.

They make use of tannin adhesives in plaster to delay the hardening time of the plaster. Another material that makes the plaster a retarded one is the borax also known as sodium borate or the retarded salt. The alum functions in the opposite direction since it is 1% of retarding function and 1% of expediting function for the hardening.

Lime extracted from the soil of chlorine and calcium:

If the amount of the lime extracted from the soil of chlorine and calcium is high in the plaster, it will reduce the time of hardening. Addition of some materials such as plant fibers, bagasse or rice bran or pieces of straw make the plaster light and leads to emergence of some problems such as color composition, reduction of pressure resistance and porosity.

They probably add some materials such as light mineral and organic grains for insulation or mix the plaster with a certain type of the soil known as the diatomaceous earth to increase resistance. The diatomaceous earth increases the resistance, makes the plaster light and creates some smooth and fine surfaces.

‌ Diatomaceous earth

Diatomaceous earth is made of the fossil remaining of unicellular algae known as diatom. As the result of its comprising elements, this type of earth is found in various colors such as white, gray, yellow and red.

Flat plaster

Any material added to the plaster aiming at increasing its strength results in having flat plasters. These materials include tannin adhesives and plant fibers.

Canvas layer

The canvas layer is stretched to a great extent on the ground and reduces the penetration of other materials. After setting the canvas layer, some features would emerge such as reduction of suction rate of the paint by the primer, ease of work with the brush and higher preservation of transparency and tarnish of the paint.

In the process of canvas-making, the surface of the wall is covered with a very thin layer of a canvas-making material. In Iranian paintings some materials such as Arabic gum, diluted tannin, diluted solution of syrup and tragacanth, white egg and diluted oil are used for this purpose. The choice of these materials depend on the painting techniques. For instance, tannin adhesives are used for oil painting or watercolor painting. Sometimes, the canvas-making material is colorful. For instance, the white primer was added to the red ground during the Safavid period to stick the gilded materials.

(Porous openings and mechanical connections, chemical links, connection between the surface and the paint in terms of polarity)

Paints uses in murals are the coating paints. They are composed of some parts. The main parts are pigments and resins.

Pigments

Pigments refer to mineral particles that exist in different shapes such as oxides, sulfides, carbonates or in natural forms in mines.