Fines are an important factor in the papermaking industry with respect to their influence on the mechanical properties of paper. A procedure offering the possibility to produce handsheets with a constant amount of fines, as well as the determination of the fines content, is of great importance in evaluating the influences of different types of fines. In this work, a method based on a white water circulation system and fiber morphology characterization using a flow cell was evaluated. Three different wires for handsheet forming were studied (120-mesh, 325-mesh, and 500-mesh), and the 325-mesh wire was chosen for further trials. Using the 325-mesh wire, a constant amount of fines was achieved after discarding seven handsheets. This method allows reliable evaluation of the effects of primary as well as secondary fines and a cellulosic additive on handsheet properties. Keywords: Primary fines; Secondary fines; Fines content; Fines retention; White water circulation; Handsheet forming; Paper propertiesContact information: a: Institute of Paper, Pulp and Fiber Technology, Graz University of Technology, NAWI Graz, Inffeldgasse 23, 8010 Graz, Austria; *Corresponding author: rene.eckhart@tugraz.at INTRODUCTIONAmong the different components and fractions present in chemical and mechanical pulps, fines have been a focus of recent research. The effect of fines on paper formation and sheet properties has been widely analyzed (Retulainen et al. 1993;Seth 2003;Sirviö and Nurminen 2004). Compared to pulp fibers, fines have a large surface area (Peterson et al. 2001), which is also an important parameter affecting various pulp and paper properties. When it comes to the evaluation of their effects on handsheet properties, a defined amount of fines is required to allow accurate extrapolation of the results.By definition of the Scandinavian Pulp, Paper, and Board test committee (SCAN-CM 66:05 (2005)), fines are the fraction of pulp that passes through a screen or a perforated plate with a hole diameter of 76 µm, representing the 200-mesh screen of a fiber length classifier according to TAPPI Test Method T 261 Cm-94 (1994). These particles can be further divided into different categories, including fines from chemical pulps, fines from mechanical pulps, primary fines, and secondary fines. Mechanical pulps, for example, contain a large amount of fines (20% to 35% by weight) that have special characteristics and impart the sheet with high opacity and reasonable strength (Retulainen et al. 1993). For mechanical fines, Brecht and Klemm (1953) content and low bonding ability and more fibrillar particles of high bonding capacity, respectively. Sundberg et al. (2003) suggested that the more fibril-like fines in mechanical pulp originate primarily from the primary and secondary fiber wall, while the more flakelike fines originate from the middle lamellae. Chemical pulps contain fewer fines than mechanical pulps. The fines content ranges from a few percent up to 10% to 12%, depending on the level of refining (Paavilainen 1992). The pri...