Rosson offers advice on tracking your tension culprit.
Reclaiming: The main cause of tension loss with wood frames stems from the way the frames are recycled for future use. Frames are generally reclaimed and processed with water and various cleaning solutions to strip off the old emulsion. In some cases, the frames are even placed in soaking tanks to soften the emulsion to the point where it can be easily sprayed off of the screen. The problem is that wood absorbs the water and cleaning liquids, which can reduce the frame's ability to withstand the stress imposed by a tightly stretched screen.
All frame types will bend inward to some degree under stress from a tensioned screen, which will cause the tension to drop. With wood frames, if the sidebars have too small a cross section, or if the frame is exposed to moisture and/or heat, the chance of deflection increases. Under stress from the screen, the wood (especially along the frame's longer sides) tends to bend inward. This condition, known as beam deflection, occurs to some degree with all frames, including rigid and retensionable metal types. But the tendency of wood frames to absorb water makes them most susceptible to this problem.
The more a frame deflects, the more relaxed the mesh will become. So a screen that was perfectly acceptable when affixed to a dry wood frame may become unacceptable after the screen goes through processing, cleaning, or reclaiming. And if the wood bends while it's wet, the frame rarely returns to its original shape, even if the screen fabric is removed.
Heat: Screens are often dried in heated cabinets or rooms that have dehumidifiers, which also create heat. The problem is that hot wood is not as strong as cool wood. Consequently, the frame's tendency to bend inward can become even more serious when it is hot. You can see evidence of this phenomena by observing old farm buildings with sway-backed roofs. Sagging roof lines are especially prevalent when the roof trusses are sheathed with tin. The reason is obvious--it is hotter under the tin roof. With wood screen frames, the presence of moisture only compounds the problem.
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