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Photographic paper for drawing .. Laboratory preparation .. Selection and use of materials .. Book of photography

Photographic paper for drawing

Enlargement paper can be classified according to the thickness of the substrate, the color of the substrate, the area of ​​the paper, the nature of the sensitive compound (speed), the advantage of the image and the contrast
(or gradation).

- Substrate:

The paper material is the substrate used worldwide, but it is a paper that resists treatments in chemical solutions and washing. The paper is carefully polished during manufacture and then receives a layer of barite (or substratum) that gives the substrate a high reflective ability that allows obtaining these very clear high lights for the images drawn on glossy paper. The normal substrate is white, but there is light yellow paper and cream paper.

- Plastic paper:

The use of saturated plastic substrate is becoming more and more widespread. This paper is known as (RC) or (PE) according to the plastic material used. Both sides of the paper are treated before spreading and spreading the sensitive compound to light. Its advantage is that the processing chemicals are not absorbed by the paper during processing, allowing for a reduction in the time required for final washing. Dimensional strength has been increased for non-plastic paper, which used to stretch during washing and especially during polishing.

- Paper grammage:

The paper substrate usually comes in two thicknesses (measured by the weight of the paper in grams per square meter): paper (100-120 g/m2) and cardboard (230-250 g/m2). The former is more easily polished (at least for non-plastics) while the latter, which is more resistant, is suitable for large-length photos if they have the same grammage. Papers that contain plastic are more resistant than non-plastics.

- Surface:

Glossy paper is preferred because it gives the purest white and deepest black. There are several materials, for example, starch is added to the light-sensitive mixture during manufacturing to give a matte surface. Thus, there is glossy paper and semi-gloss paper. However, the more matte the paper, the less capable it is of giving contrasting images.

- Nature of the light-sensitive mixture (emulsion):

There were papers whose light-sensitive mixture consisted of silver chloride (chloro paper) or of chlorobromor and silver bromor. The first papers were only used by contact drawing, so they disappeared. The second type, which gives warm colors to the images, is used for burning images, despite its slow speed. The fastest paper and the paper most used for the detector used, the general color of the image can vary from blue to warm colors, but only chlorobromor paper changes the color degree significantly.

- Gradation of the paper:

The action of papers on the same negative image is different according to their own contrast, which is called gradation. A contrasting (hard) paper gives values ​​that are highly contrasting, while a paper with a light gradation gives values ​​that are not distinct. Papers of different types are available in a full range of gradations, starting from very soft to normal soft to hard to very hard. Sometimes there is an intermediate gradation called (special) between soft and normal. However, we must note that there is no generalization in the names that manufacturers call paper. For example, a normal paper is called soft or special by another manufacturer. Gradations are also indicated by a number between zero and five. The speed of the paper varies according to the type and the gradation as well, noting that some manufacturers strive to give the same appropriate speed for their intermediate gradations of paper. Hard paper is often simpler than others.

- Paper exposure and gradation: In this graph, we see that using soft paper requires a longer exposure time to give a full range of gray values. Obtaining a deep black color in an image drawn on soft paper is more difficult than if it were on hard paper.

- Color sensitivity:

Normal magnification papers are only sensitive to violet-blue light, and some of them have a significant sensitivity to green light. If you choose a light that has no effect (orthochromatic, suitable for all colors, then light paper is usually sensitive to green and yellow radiation and also requires red light, which has no chemical effect. As for the paper sensitive to all colors of the visible spectrum and is sometimes used to extract black and white images from color negatives, it is of course sensitive to the spectrum group and requires either very strong green or - and this is better - complete darkness.

- Multi-contrast paper:

The multi-contrast paper allows all negative images to be extracted from a single box of paper. The paper of this type contains a contrasting compound sensitive to yellow light mixed with a soft compound sensitive to blue light (or vice versa) we obtain the desired contrast or gradation (gradation) by placing the filter of the appropriate color in the magnifying light beam. This can be done with some precise operations to modify the local contrast of one part or another of the image.

- Reproducing half-tones:

The gradation of the paper, although this expression is inaccurate, does not fully take into account the return of the half-tone values ​​that photographic paper can give. A certain paper One paper will give varying values ​​in the shadows while another will be better able to separate the light values ​​well (in high lights). Only experience and experimentation will tell which paper will work best for a particular effect.

Special papers:

All the compounds limited to film can be stretched on a paper or other substrate. Thus there are papers whose substrate is colored in vivid colors and some whose base is glossy: the manufacturer proposes a compound on fabric and another on opaque plastic. These papers are most often used for advertising or decoration.

- Lit paper:

Lit paper has strong contrast and gives very deep blacks and very pure whites without any half-tone. It must appear in the Lit appearance specified by the same manufacturer: there are other emulsions stretched on paper.

- Colored drawing papers:

Here the choice is limited because each manufacturer usually produces only one gradient on a white substrate. The plastic substrate is now used worldwide for color photography. There are glossy and semi-glossy surfaces, the latter being most often granular. Each manufacturer produces the colored paper that suits the characteristics of its own negative compounds. There is also a reversible colored paper that allows direct enlargement of color slides.

Fine granules 30X
Normal granules 30X
Large granules 30X

- Film granule:
The larger the granules of silver salts, the more sensitive they are to light. Therefore, a film drawn on a fast film will have more granules than a film drawn on a slow film.

- Color drawing principle by removing the colorants:

This principle, which was recently introduced in the commercial field, was invented so that part of the colorants present in each of the three layers is destroyed by light while the other part remains in the compound to form the color image. The images are drawn either on a transparent substrate for the large-dimensional slides intended to be seen in front of the light box or on a white plastic substrate for the opaque drawing that will be examined by reflection. The (positive / positive) principle gives images that have a unique stability to light.

- Use of chemicals in photography:

- Theory of development:
The light that passes through the lens of the machine to form an image on the film makes an invisible, physical-chemical modification on the latter that we call a latent or undisplayed image. The reducing agents present in the developer convert the latent image into a visible image by converting the silver bromide grains that were imprinted by the light into metallic silver.

The function of the developer is to convert the silver bromide grains that were affected by the light more quickly than those that were not affected. The materials responsible for this conversion are called reducing agents. The most commonly used materials are ginol (metol), hydroquinone and phenidone. They are reducing agents and materials since they reduce silver salts to metallic silver while the hydrogen of the developer combines with bromine (in silver bromide) to give bromohydric acid.

Bromohydric acid stops this reducing effect of the developers that do not do their work (except for one developer) except in an alkaline medium. The developer always remains in its alkaline state and the base used in the formula then comes into contact with hydrobromic acid to neutralize its effect. The bases used in the different formulas of developing solutions are called developing accelerators: sodium carbonate, soda, caustic potash, sodium borate, etc.

But developing solutions oxidize rapidly on contact with air. The developer therefore contains an oxygen-loving substance which prevents this oxidation and of course increases the preservation of the developing solution. This protectant is always sodium sulphite. A solution containing only the above-mentioned compounds - developer, development accelerator and protectant - is unsuitable because it tends to convert the imprinted and unimprinted silver bromorate, causing a general haze on the image. Therefore, a haze inhibitor is added, most often potassium bromorate, which regulates the development and delays the appearance of chemical haze.