Good Form, Bad Form: Ground Glass and Curdled Milk
Structural properties of paper describe how various components are arranged within a sheet of paper. Global properties are assessed independent of local variations in paper composition. These include the structural properties of grammage and thickness. Local properties, like formation, take regional variances into consideration.
Formation describes fiber and filler distribution in the paper sheet. Lack of uniformity of fiber and filler distribution results in thin and thick spots in the paper that will adversely affect printability and other paper properties like opacity and strength. Especially important in printing and writing grades, poor formation greatly detracts from sheet appearance.
Formation is traditionally called “look-through,” and is often assessed by valuation of appearance when paper is viewed by transmitted light. Flocculated formation is poor or ‘wild’ formation, appearing streaked or resembling curdled milk. The opposite of flocculated formation is the desirable random formation. Random formation is closed, well-closed or fine with a uniform appearance in texture, similar to ground glass.
However, visual appearance is not equivalent to structural uniformity and formation quality assessed with the human eye is flawed. The human eye is sensitive to variation in light intensity and cannot distinguish subtle variations in the distribution of material. For evaluation purposes, formation can be better defined as the small-scale grammage variation in the plane of the paper sheet.
Scott, William E., James C. Abbot, and Stanley Trosset. Properties of Paper: An Introduction. Atlanta: TAPPI, 1995.
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