Jim, It is important to remember that I said that most cracks are caused be ether compression set or compression failure. Compression set will make the board less wide across the grain and make the board more susceptible to tension across the grain (cracks). Ridges are clear indication of compression failure in the area of the ridges. That is some of the cells have been displaced enough to cause damage. Where there are multiple ridges it is likely the board has undergone some compression set as well (the board is smaller across the grain). An example of this is in my shop, a S&S D that had a replacement soundboard installed in the late 50s. Compression ridges at most of the glue lines and elsewhere. No sign of cracks. This board shows no crown even in this humid weather. Cracks are not the only symptoms of the problem; over compression can cause premature collapse of crown. I just did some quick calculations about the amount of tension and compression due strictly to curving the board into a 60 foot R. This disregards compression due to wood movement. Using a measurement of 48 inches representing the longer ribs across the grain the difference between the 720 inch radios of the bottom of the board and the 720.357 inch radius of the top of the board is .025 inch. That is, if the bottom of the board measures 48 inches and is curved to a radios of 60 ft. the top of the board will measure 48.025. This is equal to .052%. In this situation we would divide this in halve so we have .026% of tension on the top of the board and .026% compression on the bottom of the board. Since wood’s elastic limit is approximately 1% I think we can safely disregard these small figures. Also it has been my experience as well that compression ridges will most often appear on both sides of the board especially in more severe cases. I do not remember seeing cracks that do not go through the board. In my opinion compression set is the biggest factor in soundboard ridges and cracks. The board is confined mostly by the ribs. Considering that we have stress due to downbearing (about 20% of the stress to elastic limit) a soundboard can only safely be subjected to a change in EMC of 5%. If we rib the board at 5.5% EMC it can live in an environment between 27% RH and 59% RH. One of the reasons the problem is found at the glue line is that some glues can affect the hydroscopic properties in the glue line area (use hide glue instead). Another is that often too much pressure is used in making the panel and the wet wood cells near the glue line are damaged (be careful). Still another is that miss-matched grain from individual boards in the panel can add additional stresses (be selective). Nevertheless compression set and compression damage is the root cause, only the glue line area is weaker or more susceptible. John Hartman
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