Slab Flattening Issues

[Ronald Blue]

I have a friend with a sawmill business. In his efforts to grow the business he purchased a slab mill. It works well for sawing out the slabs from large logs. I don't recall the capacity but believe it's 50" or possibly over. He slabbed out a white oak log and dried them in the solar kiln. So far so good....sort of. They cupped and twisted more than expected. So by the time the 2-1/2" thick slab cleans up it was more like 1-1/4". Not what was desired. Learning curve here for everyone obviously. So we ripped the remaining slabs with his track saw and that minimized the loss. However I have been having a large issue with the slabs moving. What I mean is I get one side usually 75% flattened and flip to work the other side. It varies on how much stock is needing to come off but the las one was about 3/16" and it was cleaned up. However the opposite side is now curled upward and it will rock end to end. The amount is less than an 1/8". The side I just surfaced is still flat. I have flipped them and cleaned up the other side again and it seems to be stable at that point. Both sides are flat then. We have already determined that the slabs are needing to start out at least 3" and possibly more. That's for any we might do in the future. Any thoughts on what's happening as I mill these? AS info I am doing the flattening on a CAMaster Stinger III SR-48. It has a 4' by 8' table. Other than shimming to even out the twist etc when setting up the slabs are floating with only blocks to keep them from being able to scoot on the table. The ripping the slabs has helped with not ending up with such thin pieces but no effect on the end to end curling. Suggestions?

[Andrew Hughes]

My suggestion is the slabs need to rest.
I recommend minimum 7 years the longer the better. I was once given some wood from a local professor he said came from his dad. 40 + years it was stacked and seasoned.
No fight was left in the boards it also had a smoky patina on th edges. It was a joy to work.
Good Luck

[George Yetka]

I believe the slabs dried too quickly throwing them straight into the solar kiln. I would in the future allow the logs to dry a year or 2 before cutting them up or cut them up, anchorseal the ends and stack and sticker for a year before putting them in the kiln.

[Jim Becker]

All that you describe is related to moisture and probably some to internal tension. Slabs take a very long time to shed moisture from their cores due to thickness and width. There's a reason that makers who use slabs a lot have large drying stacks and barns...Nakashima is an example. That material is left to condition for many years before it gets in line to be considered for a project.

[Jonathan Jung]

A pinless dual-depth moisture meter is essential. I have a Lignomat SD. Wouldn't mess with slabs without it, unless I knew they were air dried for 1 year per 1" thickness, before kilning.

[Kevin Jenness]

There are plenty of possible issues at play. White oak is notoriously difficult and slow to dry, especially thick stock. If your friend is new to kiln drying there are less risky species to work with. Monitoring and limiting the moisture loss rate is crucial, even in a solar kiln. Cupping and twisting while drying can be alleviated somewhat by weighting or compressing the load, but material from leaning or crooked trees and slabs cut just off the pith are problematic. If the material is springing while milling that suggests a moisture gradient still exists or there is growth related tension in the wood.

A pin type meter with a slide hammer is useful for kiln operation to measure moisture gradients, but it can be hard to drive the pins in some woods and you can monitor samples adequately by weight. Pinless meters give an average reading throughout the wood thickness and I am not sure where to source one for thick slabs- Wagner and Lignomat appear to offer units scanning up to 3/4" deep. They will help identify "hot pockets" of higher mc which can be an issue with slabs.

As far as securing the slabs on the router, you can use hold-downs and program the toolpath to avoid them or screw down the ends using angled or countersunk fasteners, sacrificing a bit of length.

[Richard Coers]

White oak is about the worst domestic species to dry. Lots of mills don't even like to cut it 8/4 because it is so difficult because of the tyloses in the cells. Tyloses are balloonlike outgrowths of parenchyma cells that bulge through the circular bordered pits of vessel members and block water movement. The presence of tyloses in white oaksmakes their wood watertight, which is why it is preferred in casks and shipbuilding to red oak. If your friend has owned his mill any time at all, he must know how much wood moves when it's cut wide and even worse when it gets close to the pith.

[John TenEyck]

I just have to gang on here. Solar kilns of the Virginia Tech type are designed to dry 4/4 red oak without much problem provided the kiln is fully loaded. But once you go thicker you need to carefully monitor the drying process not to exceed the maximum allowable daily drying rate. The maximum allowable drying rate for white oak is lower than red oak, so the kiln already is borderline too fast for 4/4 white oak. So it's off the charts too fast on 8/4 white oak. It can be done, but only by covering off a good portion of the glazing to limit the solar gain to stay under the maximum drying rate allowed.

Look at EBAC's website for allowable drying rates vs. moisture content and thickness. White oak is incredibly low.

I air dry 8/4 white oak for 2 years before putting it in my solar kiln. Some plain sawn boards will cup during air drying. That's just the nature of the beast and happens on most wide stock with most species. There's nothing you can do about that except don't make wide plain sawn boards. Rift and quarter sawn wood is inherently more stable. Anyway, air drying will get it down to about 14% average moisture content and I can safely dry it the rest of the way in the solar dryer without issues. Without issues means without bowing, twisting, etc.

John

[Richard Coers]

A pinless dual-depth moisture meter is essential. I have a Lignomat SD. Wouldn't mess with slabs without it, unless I knew they were air dried for 1 year per 1" thickness, before kilning.

That 1 year per inch is so deceiving. It's an estimated average, how much more vague could it be? Thick white oak is probably at the top of the time curve for air drying, requiring much more than 1" per year.

[Kevin Jenness]

If I were in the slab business I would be looking at a vacuum kiln, but I am sure you can destroy wood just as easily if not following a proper schedule.

[Richard Coers]

If I were in the slab business I would be looking at a vacuum kiln, but I am sure you can destroy wood just as easily if not following a proper schedule.

Very hard to destroy wood in a vacuum kiln since the temperature rarely gets above 90 degrees F. The problem would be in wood that has different densities like crotch figure. The whole slab could be dry, except for the crotch figure and you could still have movement issues.

[Ronald Blue]

I only have a pinless meter. I forgot to mention that the moisture has been consistently below 10% anytime or place I have checked. The wood seemed to warp and twist worse after removal from the solar kiln. I had looked at them when he hadn't yet taken them out and they seemed to change a lot after removal. I don't disagree that the slabs may need different handling and drying. This is just a direction he's attempting to expand into. He has a steady pallet customer that buys about 200 pallets bi-weekly. He has a full time job but is trying to build a side business with his son. He has very little invested in these logs because a local tree service brings them to him. I suspect that some of the issue here is with a slab the grain structure varies tremendously from the edge to the pith. It's been frustrating to say the least. I'm not sure I want to invest the time into another batch. Retired or not they have been a pain in the hind end. The climate is consistently at 72 during the summer in my shop so it's not the temperature swings summer brings causing the issue. I appreciate the input thus far. When I finish with the pieces currently in process I will be done hopefully for a long while. I think maybe the best thing from your thoughts is to let them dry for maybe 2 years without any kiln time and then try giving them some oven time. The largest pieces I attempted were about 36" by 72". Definitely an educational experience for both of us.

[Richard Coers]

I only have a pinless meter. I forgot to mention that the moisture has been consistently below 10% anytime or place I have checked. The wood seemed to warp and twist worse after removal from the solar kiln. I had looked at them when he hadn't yet taken them out and they seemed to change a lot after removal. I don't disagree that the slabs may need different handling and drying. This is just a direction he's attempting to expand into. He has a steady pallet customer that buys about 200 pallets bi-weekly. He has a full time job but is trying to build a side business with his son. He has very little invested in these logs because a local tree service brings them to him. I suspect that some of the issue here is with a slab the grain structure varies tremendously from the edge to the pith. It's been frustrating to say the least. I'm not sure I want to invest the time into another batch. Retired or not they have been a pain in the hind end. The climate is consistently at 72 during the summer in my shop so it's not the temperature swings summer brings causing the issue. I appreciate the input thus far. When I finish with the pieces currently in process I will be done hopefully for a long while. I think maybe the best thing from your thoughts is to let them dry for maybe 2 years without any kiln time and then try giving them some oven time. The largest pieces I attempted were about 36" by 72". Definitely an educational experience for both of us.

Wood moves from humidity, and humidity changes with temperature. It does this every season, forever. When you put finish on it, it just slows down the movement, it's never eliminated. Any time wood moves, it's because of moisture. If it moved when it was brought out of the kiln, it was still wet. It could be the skin is 10%, but I'd be fairly confident that the center was not. A pinless meter will not reach to the center of 10/4 white oak. Oven time? White oak and an oven sounds like sterilized firewood to me. I don't know what grain structure means to you, but every inch of wood has the same cellular structure and grain means nothing to lumber people. Grain is simply done by how it is sawn, it's not a feature of a tree.

[Tom M King]

There are people that know how to dry White Oak slabs properly. I'm not one of them. I suggest you PM Scott Smith. Anything less than asking someone who actually has plenty of experience is just guesswork. First, they have to be sawn to get the best pieces out of the log, not just slice it up.

[John TenEyck]

I only have a pinless meter. I forgot to mention that the moisture has been consistently below 10% anytime or place I have checked. The wood seemed to warp and twist worse after removal from the solar kiln. I had looked at them when he hadn't yet taken them out and they seemed to change a lot after removal. I don't disagree that the slabs may need different handling and drying. This is just a direction he's attempting to expand into. He has a steady pallet customer that buys about 200 pallets bi-weekly. He has a full time job but is trying to build a side business with his son. He has very little invested in these logs because a local tree service brings them to him. I suspect that some of the issue here is with a slab the grain structure varies tremendously from the edge to the pith. It's been frustrating to say the least. I'm not sure I want to invest the time into another batch. Retired or not they have been a pain in the hind end. The climate is consistently at 72 during the summer in my shop so it's not the temperature swings summer brings causing the issue. I appreciate the input thus far. When I finish with the pieces currently in process I will be done hopefully for a long while. I think maybe the best thing from your thoughts is to let them dry for maybe 2 years without any kiln time and then try giving them some oven time. The largest pieces I attempted were about 36" by 72". Definitely an educational experience for both of us.

10% is more than dry enough for slab work. The question is, is it 10% (or any value) across the thickness. The wood needs to be within a few percent moisture content across the thickness of the slab to be stable when you surface it. Cut a slab you don't care about, at least 12" from the end, and measure the moisture content across the thickness to find out how consistent it is. A couple percent range is ideal, but you can tolerate at least 5% as long as you remove equal amounts off both sides at the same time when you work it.

My advice is to get some walnut logs. Walnut is far more stable and much easier and faster to dry, and more valuable as a live edge slab, or in most any form, than white oak. Ash is another great choice to learn with, though not nearly as valuable. For thick slabs, mill them at least 1/2" thicker than your final desired thickness. Mill them in the Fall, Winter, or early Spring and let them air dry for 2 years, then put them in the kiln. You still need control in the solar kiln, but it's not nearly as easy to ruin them as it is with green wood. Read and follow the drying guidelines from Virginia Tech as to the maximum drying rate per day.

Air drying has its own set of risks. Milling in the late Spring or Summer presents the highest probability of drying too fast and causing warping, cracks, etc. so I avoid making slabs during that time. If you must mill during that time, wrap the wood racks with landscape fabric to slow down the drying. The most important thing I've found about air drying is to rack the wood in the shade and keep a waterproof roof on it. Slow is good. You'll be waiting 2 years anyway, what's an extra month or two?

The Forest Products Laboratory has some good info. on air and kiln drying.

Good luck, patience, and diligence.

John