Sometimes a veneer may have what is called buckle, wavy spots or warp – a general distortion of the sheet. This is most commonly caused by tension wood, irregular grain, and/or non-uniform drying.

Tension wood:  Tension wood shrinks more in a longitudinal direction than normal wood during drying. This non-uniform tension wood shrinkage is probably the major cause of buckle in dry veneer.

Veneer with tension wood often has a fine fuzz to the surface rather than the more characteristic smoothness. This fuzz, which can create a silvery look, results because the tension wood fibers are weak and so do not cut easily, but rather tend to bend over. A cooler heating cycle will help strengthen these fibers, but will not affect the uneven shrinkage.

Irregular grain: Another cause of buckle is irregular grain found around knots and burls, and wavy grain in species like birch.

Non-uniform drying: Differences in the original moisture content within a sheet of veneer as well as differences in the rate at which a sheet will dry creates variation in moisture, which means stresses within the sheet and buckle. Likewise, uneven drying conditions in the dryer can affect moisture contents and buckle.  

Possible solutions
Obviously, if the logs have tension wood, which is often site related, there is little one can do. Here though are a few suggestions:

• Heating logs or flitches to 180F may relieve some growth stress, although this also may affect color. 
• Use proper lathe or slicer settings.
• In a continuous dryer, oftentimes the ends of the veneer sheets are overlapped several inches to prevent rapid drying of the ends.
• Avoid removing hot veneer from a dryer if the veneer still has moisture variations. A brief cooling prior or equalization period (sometimes called conditioning) prior to leaving the dryer and while the veneer is still held flat, does help.
• Avoid having a large amount of layers of veneer in a press dryer.

Source: Gene Wengert, “The Wood Doctor,” Read more columns at woodworkingNetwork.com/genewengert.