Paper Quality Parameters - Traditional FineArt
Quality has been our priority since 1584. Naturally, a company that has been around for more than 400 years ago manufactures its products to the highest quality standards. Without this strong focus on high quality paper, such a long company tradition would not have had a firm foundation. Our current leading position in the international market is obviously due to a commitment to deliver top quality products “Made in Germany”.
Raw Material Composition
The raw material composition determines to a large extent the quality of paper. Both the appearance and even more so the strength of paper depends largely on the quality of the raw material used. It also determines the type of paper. Thus rag paper, half-rag paper, wood-free paper and wood-containing papers are distinguished. Rag paper contains 100% linen and/or cotton fibre and is therefore the highest quality and most expensive paper. Half-rag paper usually contains 50% rag fibre and 50% cellulose. There are also different grades of inexpensive paper made of 25% rag fibre and 75% cellulose.
Paper surface (appearance)
The paper surface structure is an important quality parameter which considerably affects subsequent performance characteristics. Each paper is double-sided, i.e. the side which during production was in contact with the wire is called the wire side (bottom side). This side also bears the wire mark and is slightly more uneven. In the case of coloured papers, this side tends to be darker as pigments are deposited on the bottom.
The upper side is called the felt side or the right side, as it is the first to come into contact with the felt. It is smoother and generally brighter as fibres can be freely arranged on this side. It also contains more fillers. Surface smoothness - both obtained in the machine and during glazing - is also significant.
Clarity, opacity and transparency
Clarity indicates if the paper is coarsely ground or finely ground. On the one hand, opacity is related to paper thickness and on the other, a high filling agent content has a direct effect on this paper characteristic. Transparency is an undesirable characteristic for many paper qualities, with the exception, however, of tracing paper or paper for detailed drawings. The more transparent these qualities are, the higher their value. This property is particularly important for photocopies and microfilms of drawings.
Sizing is especially important for writing and drawing papers, but also for other paper grades. The role of paper sizing is to bind fibres and filling agents. It must be uniform and dosed so that when ink or drawing ink is applied, the lines are clean and there is no bleed. There are two types of sizing: stock sizing and surface sizing. In the case of stock sizing, the size is added to the stock already in the vat. In the case of surface sizing, the size is applied only when the paper surface has been dried. Insufficient, poor sizing can be recognized by visible jagged lines often bleeding through to other side of paper or by picking, loose fibres on the paper surface.
A feature of strongly-sized paper is a characteristic coat created when the paper is soaked.
Depending on paper application different testing methods are used to determine its strength, such as breaking strength, tensile strength, breaking length, elongation, tearing strength, folding resistance and stiffness.
Grammage and thickness
Grammage is defined as the weight per square meter and expressed in gsm. Grammages of up to 200 gsm are considered to be papers and from 200 gsm upwards they are referred to as paperboard or low-quality board.
Thickness of paper, measured in microns, is a significant property related to grammage. This parameter defines if the paper is a compact paper with a lot of fillers or a high volume paper.[mm1]
Almost all paper grades show some grain as fibre orientation corresponds more or less clearly to the direction in which the wire moves.
The direction in which the paper web moves on the paper machine is referred to as the machine direction. When paper is soaked, it stretches more clearly in the cross than in the machine direction.The paper grain is responsible for or helps to explain the following effects:
1. tensile strength is higher in the cross direction than in the machine direction.
2. stretching is higher in the cross direction
3. wet paper is curled with the axis of curl in the machine direction
4. the change in dimensions between the machine and cross direction depending on air humidity is about 1:10.
For the reasons given above, paper grain is extremely significant during further processing. In the production of blocks, the machine direction must be parallel to the spine edge of the block. Otherwise, the moisture of the adhesive may cause stretching and, as a result, curling of the block.
The grain can easily tested in practice by tearing the paper on the long and on the short side. If the paper tears relatively cleanly and in a straight line, it means it is torn in the machine direction. If the line is ragged and irregular, the paper has torn in the cross direction. If the test described above is not conclusive, place a piece of paper on a wet cloth and after some time observe the direction of waves in the machine direction.
Ageing of paper
Resistance to ageing of different paper grades depends primarily on the quality of raw materials. In the case of products with a short life cycle, such as newsprint, packaging etc., this property is not very significant. Everybody has certainly had an opportunity to observe a clear yellowing effect of a daily newspaper under the influence of sunlight. This phenomenon is caused by high wood content in the paper. Paper ageing is an ongoing process occurring in libraries and museums and in homes. Each book is affected by this slow process until, eventually, it is totally destroyed. This stealthy process is caused by aluminium sulfate, which is added to fibres in order to precipitate resin size.
Rosin soap, made from resin acid and lye, is easily soluble in water. Without the addition of aluminium sulfate, the resin soap would be washed out of the product and no paper sizing would take place. Aluminium sulfate creates resinate which surrounds and tightly binds individual fibres. During this process, some free acid (sulphuric acid) or acid salt remains in the paper. Cellulose consists of particles which are made up of many glucose units linked together like a chain of pears. It can be also be called a polysaccharide. As they react with the free acid, cellulose fibres decompose in the presence of water (this reaction is called acid hydrolysis). However, saccharide which is thus created, is an excellent nutrient medium for microbes and fungi. All these effects can be observed in the following way:
After some time paper is no longer white and is more susceptible to tearing. As the process proceeds, very old books fall into pieces when their pages are turned.
Paper stored in slightly humid conditions starts to smell musty due to microbes. Stains and holes are visible. This process has caused many books and works of art to become lost beyond repair.
Although paper decomposition described here has been known for many years, resin sizing continues to be the most commonly used method. Undesirable side effects can only be prevented by using the so-called neutral sizing, which is the method applied in the production of all the Hahnemühle papers.
This sizing method has been possible for about 20 years and is used to make exceptionally stable and ageing resistant paper qualities.
To prevent ageing , all fine art papers contain calcium carbonate as an alkaline buffer and have a pH value of at least 7. All the Hahnemühle paper grades have a calcium carbonate buffer of at least 4% and a pH value between 7.5 and 9.5.