Canvas

A canvas is a fabric made of linen, which is on or stretched on a wooden frame in a supporting frame. She serves on numerous painting techniques, such as oil painting and acrylic painting, as an underlay.

• 5.1 Light primer
• 5.2 Gypsum reasons
• 5.3 paper covering

General

The advantages of screens over other visual media such as wood, metal and walls are the low cost, low weight and flexibility ( roll up for transport). Disadvantages are lower stability and the risk of breaking the paint layer. A texture can be intentional effect, but also be perceived as a disadvantage.

Suitable substances

For the painting almost all tissues, even synthetics are. As the fabric structure has a significant impact on the material and creative effect of the painting, this choice should be met with the same attention as the rest of the painting process. The different eras had their different preferences. In the older art was preferred fabrics made ​​from hemp fiber, which were very robust. The nodes that are found in every hand-woven fabric, were beaten flat with a hammer. With multiple surface sizing they were repeatedly sanded in between to smooth out to the surface as optimally as possible.

With industrialization, with its mechanically woven fabrics, cotton fabrics to spread more. In the modern painting was found back to the hand-woven hemp fabrics and deliberately set a structure as a design tool.

Stringing the canvas

Stringing of screens requires a certain skill and practice. The cloth is stretched on a wooden frame. It should be harnessed always diagonally crosswise, ie first the top left corner, then down and then right on. The wedges that have modern frame on the back, does not serve to correct botched covering, but to compensate for voltage changes due to changes in humidity and temperature. Take caution before wedging out of the prepared canvas, as a possible surge can not be undone.

Stringing the canvas

Unsized

The fabric is stretched, it is treated with a so-called surface sizing, that is a very dilute solution of glue. Nowadays there are modern binders such as acrylic which are very " good-natured ". The old technique used bone glue, its processing is somewhat more difficult. Bone glue must be carefully heated, preferably in a water bath so it does not burn. Sometimes these smells very unpleasant.

In bone glue must be taken in exactly the right mixture of water and glue. Is it too much glue to develop during drying at high voltages which can lead to breakage of the canvas. The ratio of glue ( in dry form ) should not exceed in weight proportions 80/1000 on water. Thus to one liter of water a maximum of 80 g of glue, at the first surface sizing even just 40/1000 parts.

Other possible ingredients and admixtures

Since the bone glue further remains water soluble after drying, it is unsuitable for water-based painting techniques. To reduce the water solubility, the glue alum ( potassium aluminum sulfate) can be added. This causes precipitation of the protein content in the glue and ensures an additional bond by protein chains that dry insoluble later - comparable with the binder casein ( casein ).

It can also be added to eggs; the result is an egg tempera. This should be dealt with very sparingly, as egg tempera is not as flexible as glue and quickly tend to break. This disadvantage plays on rigid image carriers such as wood slats no matter where it can then also be used without hesitation. This egg tempera primers need to fully dried longer than pure glue reasons. While they are just as fast " smudge-proof " (that is, you can already wipe over with the dry hand), but it takes longer, until they are insoluble in water.

Occasionally honey is mentioned as an additive to enhance the flexibility. But This is not recommended. The (supposedly) increased flexibility arises only in that the honey water binds longer, but has an insulating effect on the overlying layers of oil paint.

The primer

Before the screen can be used, it must be primed. While there are some examples of recent painting, was at the surrender as a design means it consciously, but it is not advisable. The primer used for preservation, and a light foundation enhances the brightness of the colors. Unprimed materials are brittle faster.

Rembrandt preferred a dark brown undercoat, which gives his paintings the dark warm atmosphere. Rubens go preferred the white plaster base with bright Blaugraulasierung.

Light primer

Glue is almost colorless. For a white ground is therefore mixed the primer to white pigments. Here offers chalk, zinc white and titanium white. The formerly popular but toxic white lead is no longer used, instead of titanium white is usually preferred, which has a better coverage. In addition, titanium white is chemically inert in contrast to white lead. As titanium white is relatively expensive and large amounts are required for the primer, it is sometimes also stretched with the cheaper zinc white, or even cheaper chalk. The chalk should be before " eingesumpft " in water for about 24 hours because it has a powerful suction that may otherwise adversely affect the drying behavior.

The more chalk is used, the absorbent is of the ground. This can go so far that the entire binder of the paint is aspirated and the color looks very dull and partly " abkreidet " (that is, the color is not " smudge-proof "). Even after the complete drying by the color can remain in contact on the hand, because there is not enough binder is present to form a closed Malfilm.

To apply the white primer to paint rollers have proved successful.

Depending on the primer is to be noted that the primer coating shrinks during drying. This is important during stringing of the frame (if you primed the canvas after stringing ), since it is precisely in large formats by drying the primer, the pressure on the frame can be so great that breaks this.

Gypsum reasons

In the much -mentioned reasons, gypsum is not stucco plaster or the like, which sets as after addition of water, but to chalk reasons.

Paper covering

What is meant is not that the paper is attached directly to the wood frame, but on the fabric covering, almost as a primer with all its advantages and disadvantages, the paper has, for example, the strong tendency to yellow when exposed to sunlight. This is a special technique which is not very common. Noteworthy it is really only in the context of the Dada movement and its collages.

Insulation

Before the actual painting process begins, even a so-called insulation is often applied. This has on the one hand the task to regulate the absorbency of the canvas, and for other design reasons. So Rubens used, for example, deliberately irregular streaky gray-blue insulation. Anyone who has ever tried on a bright white ground, the sketch is to create know how hard it is to find the right proportions, especially in large-scale paintings. Therefore, the uniformity is broken so aware. As all material can be used, which means " thin " enough ( see below " fat to lean " ) and tends little to yellowing.

" Fat to lean "

A rule of thumb that should always be considered in the picture build (and that affects the whole picture - not just the primer ), reads: " fat to lean ." "Grease " is the binder- rich and " lean" bindemittelarm. So always a binder -rich layer to a poorer binder layer. For two reasons: firstly, to prevent tension between the image layers, and on the other hand, to ensure good adhesion. Also between the binders there is a fat - to - lean hierarchy, so are the water- soluble and oil-soluble lean the fat. Fat binders expand when drying out physically; Thus, oil paint, for example. The skinny on the other hand are contracted; very extreme, for example in casein tempera. If one were to try to apply a casein on an oil color, it would roll off to one side and on the other hand (if you, for example emulsified by addition of egg and thus still brings to adhere ) after a short time under the expansion of oil paint tear.

Is a paint layer even get too fat to be able to paint over it even further may, one should not shy away from the work and they pay off by sanding or stripping back gently until you come to a reason that is viable again.