Troubleshooting Veneer Splits And Pops
Glue type and humidity changes could be the source. April 14, 2005
We manufacture furniture. We use Beechwood substrate and different kinds of veneers. The problems are mainly splitting of veneer, creating fine white cracks and occasionally bubbles are formed. The substrate is dried to about 12%. We think the problems are caused mainly by poor adhesion and possibly high humidity content. We changed the glue and we think we should dry the wood to 6%. Worth mentioning, the problems arose about 6 month from manufacturing that took place in a relatively dryer climate. Please advise if we are on the right track or if we should consider other measures.
From contributor A:
Certainly your veneer and substrate should be acclimated together. I think 7% MC for both would be ideal.
From contributor B:
Iím not a marquetry expert but if moisture was the main problem I would expect structural stress cracks rather than what sounds like a finish problem. What are you using for glue and finish?
From the original questioner:
We are using Nitrocellulose for the final clear. We are using hide glue. In much fewer occasions, we also have the veneer splitting. If moisture change is the reason, I would imagine that the clear will eventually crack irrespective of how strong the bond is. We do not fully understand the mechanics of the problem to determine which layer is actually moving. If we apply additional final coat, will that take care of the current problem?
From contributor B:
I would not put on any additional coats until you found the problem. Several factors could be your trouble. It is always a good idea to have your materials as close to 7 percent as possible and to apply final finish to the entire piece at that level. In the gluing process I don't know what effect the water in the glue has on the veneer, it depends on your mix, temperature, and product. Fresh, compatible finish products should be used. Some top coats don't adhere well to fillers with stearates although they make the best fillers. Some will mist spray the first coats to help keep from over wetting the filler. Glue seeping through the veneer can also cause adhesion trouble with some coatings. Too much lacquer can do some weird things. There is also a difference between products like Deft and spraying lacquer. Although lacquer can be used as its' own sealer, many prefer to use something else. Most of my mistakes were from too much finish or not enough drying time. A well known marquetry instructor is Silas Kopf. I know he has several books out.
From contributor C:
At 12%, your substrate is too wet. Is it solid beech? Do you crossband it? As you are using hot hide glue I assume your doing a hammer veneer method of applying the veneer. When I've done this, I have sometimes spritzed the good side of the veneer with a little water to balance out the moisture content while hammering (squeezing) out the hide glue - it helps keep the veneer from curling so much.
From the original questioner
The substrate is solid Beech; we crossbanded and we use the hammer method. All indications so far points to high MC that was lost later and cracked the lacquer. We will attempt drying to 7%. As far as the repairs, we will experiment by lightly sanding down the lacquer and re-applying it. Finally, when we dried the wood originally to 8-9%, four hours after it is out of the kiln, it was already at 12%. Any insight here will be greatly appreciated.
Just to clarify, we have veneer backing going across the final grain veneer. Is this what is meant by crossbanding?
From contributor D:
You need to get the moisture content lower than 12% when veneering onto solid wood substrate. You should also crossband under the marquetry veneer with a backer veneer (usually poplar but can be any veneer) as well as veneering the back side. You effectively need to make the solid substrate into plywood by crossbanding under the surface veneers. Otherwise when the wood moves it cracks the veneer and finish - especially when the moisture content is so high.
From contributor E:
I just ran across your post. Here are my recommendations:
1. You are using thermoplastic glue (hide). This is flexible. In your face you have grain orientation going in every direction and that is pulling on the glue line. You need a thermoset glue line (a rigid glue line) Use urea adhesive.
2. You do not mention a veneer backer. You could apply a sheet of Okume or poplar to the back of the veneer face first. Then press this two ply veneer face to the core.
3. Moisture content - In an ideal world both veneer and substrate should be the same. Shoot for the 7 to 8% MC range.
When you made the product in a humid environment and shipped it to a drier climate, the veneer and the adhesive pick up moisture. This causes the veneer to expand and the tension created by this expansion pulls on the soft glue line that you are using.
1. Use Urea
2. Make two ply if you can
3. Balance MC of face and core
From contributor F:
Last year I did a job for a customer; I made nursery furniture from beech. Since beech is an imported wood here and I had very little left, I used 5mm thick 8" wide glued over a MDF cabinet casing. The MC was 8%. It was left unfinished for one week and the whole outside expanded, tore out the base and rippled like sea waves. I redid whole thing, this time in 3" wide strips. This time it seemed ok but only for three months. In summer dry weather the strips shrunk about 1mm and exactly the same cracks appeared. I keep 5mm strips 8" long of the woods that I use to show to customers the different colors, textures and grains. Those made of beech, although sealed and finished on all sides, show similar strains. Two years ago I made 120 Queen Anne chairs from the same stock, to be used in Dubai - no problem at all till now. I donít have much experience with this wood but I have found that in shape of thin boards it is unstable; it absorbs and releases moisture quickly, and its ratio of expansion and contraction is very high.