REimagine Paper - May 2018

May 15, 2018

Absolute/relative moisture

Absolute moisture: the absolute moisture of the air is the maximum amount of water vapour which the air can contain before the excess water is released as dew or frost. Absolute moisture is measured in grams per cubic metre. As the temperature falls the air contains less water in grams at the same relative moisture.

To understand why the air may be drier or damper, we can consider why someone's beard becomes frosty on a cold winter day, when the air seems to be dry, while water evaporates when we sit in a damp sauna.


Brightness is one of the most important characteristics of paper. Very often brightness is held to be the sole measure of quality, but for overall quality the matter is not so simple. For example, opacity generally weakens with an increase in brightness.


The appearance of brightness depends on the nature of the light falling on the paper, that is the distribution of the light's wavelength. For this reason comparison of the brightness of different papers must be carried out in the same place or with the same measuring equipment. Brightness is now a days measured with a spectrophotometer which can be used in accordance with different standards (DIN, ISO, SCAN and TAPPI). The same apparatus can be used to determine paper shade and opacity. The best meter for brightness and shade is, however, the human eye.

Bulk expresses the specific volume of a material. Bulk is the inverse of density.


bulk = 1 / density = (cm²/g)

In the paper trade bulk is a more commonly used measure than density to indicating the "compactness" of paper. Bulk can be calculated by dividing the thickness of a sheet by its basis weight. In some connections, bulk is used to express the thickness of paper or board according to the following equation:

bulk x basis weight = thickness

For example:


Bulk Basis weight Thickness

1.3 100 gsm 130 microns

1.6 80 gsm 130 microns

This is done particularly when choosing book printing paper and packaging.


Density is the specific weight of a material. Lead is denser than cottonwool: a cubic metre of (m²) of lead weighs more than a cubic metre of cottonwool. The density of water is 1000 kg/m². When the density of a material is less than that of water, the material will float on the water's surface and will not sink. Paper density expresses how compact the paper is. Paper density is calculated from the basis weight and the thickness as follows:

Density = w/thickness = (gsm) where w is the basis weight.

Gloss expresses the amount of directed light that is reflected in a certain direction. The glossier the paper, the better the image reproduction.

The basis weight or grammage of the paper indicates how many grams one square metre (1m x 1m = 1m²) weights. The DIN size A0 is 841 x 189 mm, which is almost exactly one square metre and equivalent to 16 A4 sheets. The grammage is obtained by weighting 16 A4 sheets.

Newsprint normally weights 45 gsm and copying paper 80 gsm. If the weight is 200 gsm or more the paper is called board. Board can weight 400 - 2400 gsm. The grammage of an ordinary writing pad backing board is 400 grams.

Paper is a very living material. Variations in humidity and temperature have a great effect on it.

Therefore it is important to know how the paper will react to the characteristics of the surrounding air. If the surrounding air is more humid than the paper, the paper fibres absorb the humidity and swell. If the air is dryer than the paper, the paper fibres absorb the humidity and swell. If the air is dryer than the paper, the paper fibre releases its own moisture and shrinks.

Inkjet printer
Creates images by ejecting tiny droplets of ink onto paper.

International organisation for Standardisation. An international organisation that has approved a range of standards extensively used in industry.

ISO 9001
An international series of standards covering quality management and quality assurance. The Finnish equivalent are ISO standards coded SFS-EN.

ISO 14001
An international series of environment-related standards.

Laser printer
A high quality image printing system using a laser beam to produce an image on a photosensitive drum. The image is transferred on to paper by a conventional xerographic printing process.

Opacity measures the transparency of paper. Paper with poor opacity is relatively transparent. Paper with high opacity is not transparent at all.

Paper grades
Paper is classified into different grades according to the end use, the pulp used and the treatment of the paper.

A term denoting a number of sheets of paper ranging from 480 to 516, most commonly 500.

The structure of the surface of paper varies in just the same way as the surface of the landscape. For example, the pinholes between fibres in uncoated paper are relatively very large like the distances between valleys and mountains, where as the glossy surface of coated paper, with its smaller irregularities, is considerably smoother. The term roughness is usually only used in relation to uncoated paper.

Thickness is measured in microns, or thousandths of a millimetre. The thickness of different paper qualities. Newsprint 45 gsm about 70 microns copying paper 80 gsm about 100 microns thin cardboard 180 gsm about 200 microns pad cardboard 400 gsm about 600 microns. In everyday use we choose paper by its basis weight and cardboard by its thickness.

The visual appearance of white paper, especially when it contains fluorescent whitening agents. It is based on reflectance data obtained over the full visual spectrum


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