WORLD PULP&PAPER 27 comparing them with the spectrum. Thus, in the future, we know that a high vibration at 6 kHz is likely caused by Yankee edge contamination. Higher frequency vibration is generally harder coating in the sheeted area, and consequently may have quality or crepe ratio implications. In summary, the skilled process engineer will use a range of techniques to make judgments about the quality of the Yankee coating. Having measured it, the next question is often “why is my coating hard” – and one which is addressed in the next section. IMPACT OF TISSUE MACHINE WET END UPON COATING LAYER The applied Yankee coating chemicals are only part of the story on our Yankee surface. Practically everything else in the tissue machine stock and process water system has the potential to end up on the Yankee surface. Some researchers put the applied coating add-on at less than 1% of the total Yankee organic layer.2 The contaminants can include celluose short fibre and fines, hemicellulose, lignins, starch, other polymeric dry strength additive, wet strength resin and others, such as inorganic ash. The influence of chemical parameters such as pH, ionic charge demand, temperature, water hardness and system conductivity, as well as Yankee heat flux and other operational parameters can all impact how these behave on the Yankee surface. In this section we will focus on the fines, fibre and ash. Short fibre, such as eucalyptus or inappropriately refined fibre can be easily incorporated into the catonic Yankee coating. Fines and ash may be added to the tissue process at various points, and in somes cases (e.g. inappropriate refining) they will be created in the system. In addition, the short and long water loops will continuously recycle these materials. A stable equilibrium is eventually reached on the tissue machine system. Thus, the net addition of fines plus fines generated must equal what is incorporated in the sheet exiting the machine system plus losses to sewer and sludge disposal; or what ends up on the Yankee, causing the issues decribed earlier. The kind of fines circulation loops we have can be depicted as a Sankey diagram, thus: Figure 3. Frequency mapping of Yankee surface defects to blade vibration spectrum. Figure 4. Simplified Sankey diagram of fines recirculation on a tissue machine.