<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> <HTML><HEAD><TITLE>Re: Plywood shrinkage??? Laminated = soundboards???</TITLE> <META http-equiv=Content-Type content="text/html; = charset=iso-8859-1"> <STYLE type=text/css>BLOCKQUOTE { PADDING-BOTTOM: 0px; PADDING-TOP: 0px } DL { PADDING-BOTTOM: 0px; PADDING-TOP: 0px } UL { PADDING-BOTTOM: 0px; PADDING-TOP: 0px } OL { PADDING-BOTTOM: 0px; PADDING-TOP: 0px } LI { PADDING-BOTTOM: 0px; PADDING-TOP: 0px } </STYLE> <META content="MSHTML 6.00.2800.1226" name=GENERATOR></HEAD> <BODY bgColor=#ffffff> <DIV><FONT face="Comic Sans MS" color=#0000ff></FONT> </DIV> <BLOCKQUOTE dir=ltr style="PADDING-LEFT: 5px; MARGIN-LEFT: 5px; BORDER-LEFT: #0000ff 2px = solid; MARGIN-RIGHT: 0px"> <DIV class=OutlookMessageHeader dir=ltr align=left><FONT = face=Tahoma size=2>-----Original Message-----<BR><B>From:</B> = pianotech-bounces@ptg.org [mailto:pianotech-bounces@ptg.org]<B>On Behalf Of </B>Sarah Fox<BR><B>Sent:</B> February 15, 2004 8:58 AM<BR><B>To:</B> Pianotech<BR><B>Subject:</B> Re: Plywood shrinkage??? Laminated soundboards???<BR><BR></FONT></DIV> <DIV><FONT face=Arial color=#000000 size=2></FONT> </DIV> <DIV><FONT face=Arial size=2></FONT> </DIV> <DIV><FONT face=Arial size=2>I'm just curious about the moisture = blocking properties of the SB finish:  How much does the finish alter the = rate of MC change in a SB?  I presume it depends on the type and = thickness of finish used.  Has anyone done any side-by-side measurements?</FONT></DIV></BLOCKQUOTE> <DIV dir=ltr><SPAN class=406452621-15022004><FONT face="Comic Sans = MS" color=#0000ff>Yes. Check the archives. Basically, I know of no finish = material other than multiple coats of epoxy that will form a effective vapor = barrier when applied to wood.</FONT></SPAN></DIV> <DIV dir=ltr><SPAN class=406452621-15022004><FONT face="Comic Sans = MS" color=#0000ff></FONT></SPAN> </DIV> <DIV dir=ltr><SPAN class=406452621-15022004></SPAN><FONT = face=Arial size=2></FONT> </DIV> <BLOCKQUOTE dir=ltr style="PADDING-LEFT: 5px; MARGIN-LEFT: 5px; BORDER-LEFT: #0000ff 2px = solid; MARGIN-RIGHT: 0px"> <DIV> <DIV><FONT face=Arial size=2>Back to the plywood...</FONT></DIV> <DIV><FONT face=Arial size=2></FONT> </DIV> <DIV><FONT face=Arial size=2>Regarding quality:  Although the = drawer bottoms may  be constructed of relatively poor quality material, = I would strongly suspect only the best materials are used in boat = construction.  In fact knowing some boating enthusiasts, who pour unbelievable = amounts of money into their surrogate babies, I'd suspect the quality of these = materials would rival what you builders/rebuilders use!  Furthermore, the = reference to boat deck plywood shrinkage is most likely with regard to = modern-day materials.</FONT></DIV></DIV></BLOCKQUOTE> <DIV dir=ltr><SPAN class=406452621-15022004><FONT face="Comic Sans = MS" color=#0000ff>The shrinkage/expansion potential has more to do with = the physical characteristics of the wood used (its compression strength perpendicular = to grain, etc.), the type of adhesive used (its rigidity and/or = its cold-flow characteristics) and the thickness of the veneers used than its = "quality." In other words, a "high quality" plywood using top-grade, relative thick, = hardwood veneers might be less stable than a "cheap" plywood using thinner = doug-fir peeler veneers.</FONT></SPAN></DIV> <DIV dir=ltr><SPAN class=406452621-15022004><FONT face="Comic Sans = MS" color=#0000ff></FONT></SPAN> </DIV> <DIV dir=ltr><SPAN class=406452621-15022004><FONT face="Comic Sans = MS" color=#0000ff>And on the starting moisture content. Wood, regardless = of how it is configured, does not continue to shrink just because it gets older. = It must start its shrink from somewhere and that somewhere depends on how much = moisture is bound in the cells of the wood. Once that moisture is gone the wood = stops shrinking. If moisture gets back in the wood expands -- that is the = opposite of shrinking -- until the cells are saturated, then it stops = expanding. In other words there will be a range within which the wood will move. = There will be a maximum dimension and a minimum dimension, both dependent on = moisture. Regardless of what you do to it, it doesn't just keep on shrinking. = Well, I guess you could burn the stuff ... it shrinks quite a bit then.</FONT></SPAN></DIV> <DIV dir=ltr><SPAN class=406452621-15022004><FONT face="Comic Sans = MS" color=#0000ff></FONT></SPAN> </DIV> <DIV dir=ltr><SPAN class=406452621-15022004><FONT face="Comic Sans = MS" color=#0000ff>Because of its cross-banded nature plywood is far more = stable than so-called "solid" wood. But there will still be some nominal movement = with changes in moisture content, but these changes will always be subject to = the basic laws that govern wood strength and wood = movement.</FONT></SPAN></DIV> <DIV dir=ltr><SPAN class=406452621-15022004><FONT face="Comic Sans = MS" color=#0000ff></FONT></SPAN> </DIV> <DIV dir=ltr><SPAN class=406452621-15022004></SPAN><SPAN class=406452621-15022004></SPAN><SPAN = class=406452621-15022004></SPAN><SPAN class=406452621-15022004><FONT face="Comic Sans MS" color=#0000ff></FONT></SPAN> </DIV> <BLOCKQUOTE dir=ltr style="PADDING-LEFT: 5px; MARGIN-LEFT: 5px; BORDER-LEFT: #0000ff 2px = solid; MARGIN-RIGHT: 0px"> <DIV><FONT face=Arial size=2>I suspect shrinkage may be the fate = under extreme conditions.  The boat deck plywood obviously gets baked in the = sun, doused with water, and so forth.  Drawer bottoms can frequently = be wiped out with wet rags -- which while not as extreme, still creates a rapid = and drastic changes in MC.  Perhaps more importantly, another dynamic = is going on in both these cases.  Deposition of water is never = uniform, whether through leakage past finish defects (boat) or through = differential deposition of water with a wet rag.  This can cause a lot of = internal stress and strain in the panel.  For instance, if a small = spot in the center of a panel is soaked with water,and the surrounding board = remains dry, the swollen center spot will be compressed within the "frame" of = the surrounding wood.</FONT></DIV></BLOCKQUOTE> <DIV dir=ltr><FONT face=Arial size=2><SPAN class=406452621-15022004> </SPAN></FONT></DIV> <DIV dir=ltr><FONT face="Comic Sans MS" color=#0000ff><SPAN class=406452621-15022004>All of this is true, but please remember that = in most plywood construction the laminae are quite thin. Thin enough that they = are no longer capable of creating much in the way of compression (to expand the = panel) or tension (to cause it to shrink). That is the whole point of plywood. = It is the relative thinness of the laminae and the crossbanding that gives = plywood its relative stability.</SPAN></FONT></DIV> <DIV dir=ltr><FONT face="Comic Sans MS" color=#0000ff><SPAN class=406452621-15022004></SPAN></FONT> </DIV> <DIV dir=ltr><FONT face="Comic Sans MS" color=#0000ff><SPAN class=406452621-15022004>Nor am I convinced by the many tales boat = owners tell of their plywood decks "shrinking." Solid teak overlays certainly do = swell and shrink and boat hulls certainly do twist and warp and I wonder just how = any of them are so certain it is the plywood decking that is "shrinking." I've = lived on and been around too many boats and have listened to way too much pure = poppycock from boat owners (many of whom should know better) about their boats to believe much of anything they have to say about their = boats without verifying it first. The most memorable tale of shrinking plywood = decking -- it couldn't be the fiberglas hull, could it? -- turned out to be the = hull that was literally coming apart. The was spreading across the beam, = ultimately splitting the joint between the hull and the fiberglas deck molding. The = plywood decking turned out to be about the only thing that was holding the thing = together.</SPAN></FONT></DIV> <DIV dir=ltr><FONT face="Comic Sans MS" color=#0000ff><SPAN class=406452621-15022004></SPAN></FONT> </DIV> <DIV dir=ltr><FONT face="Comic Sans MS" color=#0000ff><SPAN class=406452621-15022004>Del</SPAN></FONT></DIV></BODY></HTML>