Glossary of paper & pulp testing terms

The inorganic residue remaining after a sample is incinerated at a specified temperature, expressed as a percentage of the original oven-dry mass. It quantifies mineral filler and coating pigment loading.

ASTM International, a standards body whose D-series methods cover paper, packaging and related materials, widely referenced in North American specifications.

The mass of paper per unit area, reported in grams per square metre (g/m²) or, in some markets, pounds per ream. It is the single most fundamental property of a paper or board grade, and most other strength properties are normalised against it.

The maximum compressive load a complete, assembled box withstands before failure, reported in newtons. It is the closest laboratory analogue to the load a carton sees when stacked in a warehouse or container.

The diffuse blue reflectance of a sheet measured at an effective wavelength of 457 nm, expressed as a percentage of a perfect reflecting diffuser. Distinct from whiteness, which accounts for the whole visible spectrum.

The reciprocal of apparent density — caliper divided by basis weight, expressed in cm³/g. High bulk means a thicker, lighter sheet, which raises stiffness for a given grammage and is valuable in board and book grades.

Bursting strength divided by basis weight (kPa·m²/g). Normalising by grammage allows sheets of different weights to be compared on the strength contributed by the furnish rather than by sheet thickness.

The hydrostatic pressure required to rupture a clamped specimen when pressure is applied through an expanding elastomeric diaphragm. Reported in kPa, it is a composite indicator of the sheet's tensile strength and stretch, and is widely specified for liner and sack grades.

The thickness of a single sheet measured under a defined static load with a precision micrometer, reported in micrometres. Caliper governs stiffness and runnability, and together with basis weight it determines bulk.

The mass of water in grams absorbed by one square metre of paper surface in a specified time, usually 60 seconds. It is the standard measure of sizing and of a surface's resistance to water penetration.

Holding specimens at a controlled temperature and relative humidity — conventionally 23 °C and 50 % RH — until they reach moisture equilibrium, so that results from different laboratories are comparable.

The oven-dry mass of fibre expressed as a percentage of the total mass of a pulp suspension. Accurate consistency is a precondition for almost every other pulp test, since results are reported on a fixed-mass basis.

The compressive strength of a short length of corrugated board loaded on edge, perpendicular to the flutes, reported in kN/m. ECT is the strongest single predictor of the stacking strength of a finished box.

The resistance of the fluting medium in corrugated board to a crushing load applied flat to the board surface, reported in kPa. It indicates whether the flutes have been damaged or crushed during corrugating.

The number of double folds a strip survives under tension before failure, usually reported as a logarithmic fold number. It is the most sensitive indicator of fibre degradation, and so is used for permanence and ageing studies.

A measure of how readily water drains from a dilute pulp suspension, used as an index of the degree of refining. Reported either as Canadian Standard Freeness in millilitres or as a Schopper-Riegler number; the two scales run in opposite directions.

A laboratory sheet formed from a known mass of pulp under standardised conditions so that furnish properties can be measured without the variables introduced by a paper machine.

The International Organization for Standardization, whose paper and board test methods are the principal international reference and are frequently specified alongside or instead of TAPPI methods.

The volume of potassium permanganate solution consumed by a pulp sample under standard conditions, used as a proxy for residual lignin content and therefore for the degree of delignification achieved in cooking.

The direction in which the sheet travels through the paper machine, along which fibres preferentially align. Strength properties measured along it differ substantially from those measured across it.

The mass of water in a sample as a percentage of its total mass, determined by oven drying to constant weight. Paper is hygroscopic, so moisture affects nearly every strength and dimensional property.

The degree to which a sheet obstructs the passage of light, reported as the ratio of the reflectance of a single sheet over a black backing to its reflectance over an opaque white backing. It determines show-through in printed matter.

The internal bond strength between the plies of a multi-ply board, measured as the energy needed to delaminate them. Low ply bond shows up as splitting during converting or printing.

The rate at which air passes through a sheet under a defined pressure difference, reported as an air permeance or as the time for a fixed volume to pass. Porosity affects drying, printing and, in packaging, barrier performance.

The total energy absorbed when a pyramidal head is driven through a board specimen, reported in joules. It characterises resistance to impact damage in transit rather than to steady compressive load.

Mechanical treatment of pulp fibres in water that fibrillates the fibre surface and increases bonding area. Refining raises tensile and burst strength but shortens fibres, so past an optimum it reduces tear strength and drainage.

The edgewise compressive strength of a strip of paperboard formed into a ring and loaded axially. Used to predict the contribution of liner and medium to the compression strength of the finished board.

A freeness scale in which a higher value means slower drainage and therefore more refining — the inverse convention to Canadian Standard Freeness. Reported in °SR.

The surface regularity of a sheet, measured by the rate at which air leaks between the sheet and a precision-machined ring — the Bendtsen and Gurley methods being the common ones. Smoothness governs print quality and ink demand.

The resistance of a sheet to bending, most often measured as bending resistance at a fixed angle (Taber) or as bending stiffness in mN·m. Stiffness rises roughly with the cube of caliper, which is why bulk matters so much in board grades.

The Technical Association of the Pulp and Paper Industry, the North American body whose T-series test methods are among the most widely cited standards in the industry.

The mean force required to continue tearing an initial cut in a sheet, measured on an Elmendorf-type pendulum and reported in millinewtons. Tear is driven primarily by fibre length, so it responds oppositely to refining than tensile does.

Tensile strength divided by basis weight (N·m/g). The standard way to compare the intrinsic strength of different furnishes independently of sheet weight.

The maximum force a strip of paper withstands before breaking in tension, reported per unit width (kN/m). Measured separately in the machine and cross directions, since fibre alignment makes paper markedly anisotropic.

A measure of surface strength in which graded wax sticks are adhered to the sheet and pulled away; the highest grade that does not lift fibres defines the surface strength. Predicts picking during offset printing.