Last week I began a discussion about veneer checking and how to recognize it. I briefly discussed how veneer is made and then went on to talk about how movement within the wood affects the veneer. Please look back at that first installment so that you will be on the same track with me from here on.
This week let’s go back to making veneer. All that we have done to this point is to knife off the wood into thin sheets. Next, we need to join those into useable sized pieces that will be the veneer that we want.
There are a number of forms of veneer available. There are the raw strips of veneer available if you choose. These strips are then edge butted together into sheets using a special tape so that they can be glued to a substrate. These sheets are designed to be glued directly (wood to wood) to the substrate. This is generally how the plywood makers proceed. However, people who buy veneer on the market for their own application start with veneer that is thicker than the final, sanded versions we see on plywood sheets that we buy from our lumber suppliers.
Then, there are three common forms of sheet veneer available that come with a “backer” to which the veneer has been glued. Some feature a wood backing sheet that is applied cross-grained in a plywood-like fashion to improve stability. Some have a paper backing. This paper usually has a certain amount of phenolic resin impregnated into it that improves the performance of the paper.
Then, there are those veneers that have been applied to a heavier phenolic backing. Thicker than the paper, the thickness of this backer makes these very much like plastic laminate. This backing is very stable and a great barrier. Its two small drawbacks are that, because of the thickness of the backer, you will see the backer on any edge of a project. Also, forming it around a short radius may be problematic.
Now, let’s say that we have the veneer applied to the backer and the backer applied to the substrate. Let’s assume that something causes a humidity change.
What may potentially happen to the wood of the veneer? Increase the humidity and the wood portion of the veneer will tend to swell and to do so primarily across the grain. Reduce the humidity and the wood will tend to shrink in the same plane. We know that wood will do this and that engineered panels tend to move as well…but at a different rate. In fact, I would suggest that the veneer, through swelling or shrinkage, will actually try to become larger or smallerthan the backer to which it is firmly bonded.
What happens? Well, I would submit that there will be a change in the level of stress resulting from that environmental change. I can see two possible scenarios based upon the kind of glue used to bond the backer to the substrate.
If a cold press glue, a two-component glue, or a heat-set glue has been used that very rigidly bonds the veneer to the substrate, when push comes to shove and the veneer retracts, cracks will appear in the veneer running with the grain. These cracks are what we commonly refer to as veneer checking. If the slack side is facing up, the cracks may be quite obvious and may well include cracks large enough that they go all the way through the finish layer(s) as well.
If “contact cement” is used, which remains flexible throughout its life, then the stresses may become such that the veneer – bonded firmly to the backer - actually pulls the paper away from the substrate. Also it is possible, as humidity increases and if the bond is weak enough, for the wood of the veneer to swell enough that the stress will, again, pull the paper away from the substrate and form ridges…pressure ridges (much like ice flows in the winter Arctic icecap). This ridging up is known as veneer ridging. Often these ridges will ebb and flow as the humidity goes up and down. Such can be the results of changing stress.
Obviously both checking and ridging are to be avoided. Decisions need to be made regarding the gluing methods that will insure that the veneer is firmly bonded to the substrate in such a way that ridging will not happen. Also, decisions need to be made that will allow the veneer sheets and substrate to acclimate properly before they are joined together. In my experience, these two decisions are, regrettably, glossed over or ignored. After all, it is only when checking and ridging raise their ugly heads that we look back to find that smoking gun. It’s tough when we come to realize that we hold it in our own hand!
Next week we are going to go into how environmental changes can potentially affect the panels that we create. I encourage you to tune in so that you can have the opportunity to see how those environmental changes can impact the veneered panels we use.
Until next time…Spray on!
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