Cupped slab for Nakashima style bench

[Robert Johnson]

I'm a fairly new handtool woodworker and I'm building a Nakashima style bench as my first project. (I'm basically copying the George Nakashima's R bench.) I have a large 8/4 slab that my wife and I recovered from her grandparents' Pennsylvania farm about 20 years ago. I think it's oak, but I'm not sure. It's about 8 feet long and tapers from 14 to 16 inches in width. I plan to crosscut the wide end to make a leg for one end of the bench.

The 2-inch thick slab has a 3/8-inch cup and I'd like to flatten the top and leave the bottom the way it is. Both sides are currently quite rough. I can't decide whether to plane the concave side or the convex side for the top. I'm leaning towards flattening the convex side and making that the top. Are there any advantages or drawbacks for choosing this approach? I'd like to avoid causing more cupping or perhaps encourage the non-planed side to be more flat.

— Robert in Albuquerque

[David Weaver]

I'd knock the high points off of the edges on the concave side first. Figure that's the side pulling together, and you want to relieve it.

[Chris Vandiver]

I'd first look at the slab and determine which side is from the outside of the tree. This side is typically the show face and it is also the side that will cup more readily than the inside face. Nakashima was greatly influenced by Japanese woodworking and for the most part, a Japanese craftsman would never use the inside face as the show face(unless it were facing in), if you get my drift. Another thing to keep in mind is that if you plane one side down you need to do something to the oppisite side to act as a counter balance(or the slab may continue to cup). Carvers use this technique often. The cupping you're experiencing may be due to the slab sitting cupped side up for a period of time, so that it is drier than the side facing down(sunlight, etc.). As David Weaver says, the concave side is the easiest to flatten.

[Jeff Heath]

I agree with all that is already mentioned ^^^^^, and would add that a moisture meter will really help you tell what's going on within the wood. I work with slabs alot, and I can tell you that cupping is usually the result of uneven drying (unless, of course, you've got a piece of timber loaded with reaction wood). You do need to work both sides, though, if you want it to stay reasonably flat.

[Sean Hughto]

Flatten both sides and choose which is the "top" based upon which looks more appealing to your eye (grain, color, knots, live edge, etc.).

[Brian Holcombe]

I think Nakashima mostly uses the outside face as the show side.

[Prashun Patel]

It is the leg side that requires more critical flattening.

[Chris Vandiver]

It must be remembered that Nakashima had access to the highest quality slabs found anywhere(he built mainly for Rockefeller) and his lumber inventory was(is)legendary. I'm sure his meticulous drying and storage methods made cupping less of a problem, for the most part. His designs don't lend themselves to the use of unstable lumber. In fact, they rely heavily on the stability of the slab. Any cupping on the top will move the underside as well, bringing the legs with it.
What goes around comes around.

http://www.nakashimawoodworker.com/furniture/9/43

[Brian Holcombe]

His tables often utilize battens or joinery that serves as a batten.

[Jim Matthews]

A bench need not be flat, to provide comfortable seating.

If this is your first effort, solid joinery for the base
and stability are paramount. If the curve is subtle,
why not keep it?

280750

http://www.adrianmccurdy.co.uk/stools-and-benches.html

[Chris Fournier]

It must be remembered that Nakashima had access to the highest quality slabs found anywhere(he built mainly for Rockefeller) and his lumber inventory was(is)legendary. I'm sure his meticulous drying and storage methods made cupping less of a problem, for the most part. His designs don't lend themselves to the use of unstable lumber. In fact, they rely heavily on the stability of the slab. Any cupping on the top will move the underside as well, bringing the legs with it.
What goes around comes around.

http://www.nakashimawoodworker.com/furniture/9/43

A slab, especially of the scale that George Nakashima used is inherently unstable. Nakashima went to lengths to stabilize his projects without them tearing themselves apart.

I helped sell a quantity to livesawn lumber to his daughter Mira and there was nothing exceptional about the dimensional stability of the lot. It was large and nicely figured, simple as that.

Personally I'd flatten both faces and choose the show face as Sean has suggested. If this is not your cup of tea then I would machine as you see fit and watch for movement - taking several weeks to do so. Joinery is always easier when using flat, square surfaces.

[Chris Vandiver]

A slab, especially of the scale that George Nakashima used is inherently unstable. Nakashima went to lengths to stabilize his projects without them tearing themselves apart.

I helped sell a quantity to livesawn lumber to his daughter Mira and there was nothing exceptional about the dimensional stability of the lot. It was large and nicely figured, simple as that.

Personally I'd flatten both faces and choose the show face as Sean has suggested. If this is not your cup of tea then I would machine as you see fit and watch for movement - taking several weeks to do so. Joinery is always easier when using flat, square surfaces.

Nakashima has been dead for 23 years, so I seriously doubt you sold him wood while he was alive and certainly not during his heyday(1950's-1960's) when most of his work was done for the Rockefellers. The wood his daughter buys might have a different criteria and of course. really premium, old growth slabs are very hard(and expensive) to come by.

[Sean Hughto]

I'm interested to know what premium old growth slabs are like. Do they not cup and breath like other slabs?

[Chris Fournier]

Nakashima has been dead for 23 years, so I seriously doubt you sold him wood while he was alive and certainly not during his heyday(1950's-1960's) when most of his work was done for the Rockefellers. The wood his daughter buys might have a different criteria and of course. really premium, old growth slabs are very hard(and expensive) to come by.

I was very clear in my post about whom I helped sell wood to. I very seriously doubt that you read my post.
Wood moves in predictable ways old growth or less so. Large scale pieces will move considerably and at different rates from edge to edge due to grain orientation.

The more things change the more they stay the same.

[Kees Heiden]

I have made a slab table last year. I used two 40 cm wide ones and a narrow one in the middle, so the slabs weren't super wide. 4cm thick, 2.10 meters long, and no glue joint in the middle, they are about 1 cm apart. Sturdy undercarriage with thick stretchers sideways and two lengthwise.

I flattened both sides. Which was a huge amount of work with handplanes. One of the slabs decided to twist within a week after I flattened it. The other only bowed a little , which was no problem with the sturdy under carriage. But the twisted one was a problem, because it wanted to lift a leg. In the end I deepened the offending place with a gouge, so the underside is not so nice anymore. The table has been through a wet summer and is now in the middle of the dry inhouse winter climate and it behaves perfectly well.

We discussed this a bit back when I was working on the table and several people reported middle age tables made with very thick and very wide boards. These were only flattened on the top, the underside was cut out where the tables were attached to the stretchers. So, these didn't selfdestruct either.

Conclusion? Hmmm, no idea really.

[Jim Matthews]

@ Kees -

Is there a convention on orienting the slab, so that the "outside" from the tree faces
a particular direction? I know that in placing solid cabinet tops, I orient the growth rings
to resemble a "smile" or trough. That way when cupping occurs naturally,
it is toward the carcass and no gaps result.

This much movement could lift a leg right off the floor.

Is there any standard to the Medieval tables and benches?
I would be surprised to find any outside of museum collections.

[george wilson]

Boards warp concave on the "outside side" of the tree that were sawn from. They always do.

If the table was a dog table,I'd be careful of it lifting 1 leg off the floor!!!!:) Why did I type that?

[Brian Holcombe]

Even boards fit for use on pieces made for Nelson Rockefeller move in the same way all of the others do. My impression of Nakashima's work is that his process is borne out of practicality, the wood he chose for each piece shows this in my opinion.

Often times it seems like the peices he chooses for a bench, which will remain basically unsupported are much different than those which he choses for projects with more possibilities for interlocking joinery.

[Kees Heiden]

Good question about the orientation. I didn't have much choice. One board has the pith in the middle (an absolute no-no, but sometimes you get away with it). The other board is with the inside facing up. They are each others mirrors. The other sides of the boards look awfull.

BTW, this is European wallnut which is a stable kind of wood.

[Dan Hulbert]

I built a slab bench last year and had a similar issue with cupping. My solution was to smooth both sides and ignore the cup. My slab was triangular so I used three legs with a hay-rake stretcher. It was really tricky getting everything to align properly, but in the end the wife is happy with the result. It even won me a blue ribbon at the Colorado State Fair.

[Dan Hulbert]

Found a picture of my blue ribbon slab bench.

[Jessica Pierce-LaRose]

I might not have picked contrasting wood, but that's really nice, Dan! The hayrake stretcher on a slab bench is a neat detail I can't remember seeing before. The slab looks really great in that space, too. Nice job!

[Dan Hulbert]

The slab is horse chestnut, the legs are cherry, the butterfly and tenon wedges are jarrah. I filled the cracks with clear epoxy and semi-precious stone cut-offs (turquoise, malachite, lapis).

I saw the hay rake on the Barnsley Table. I think I spent an entire weekend on that alone.

[Tony Shea]

Nakashima has been dead for 23 years, so I seriously doubt you sold him wood while he was alive and certainly not during his heyday(1950's-1960's) when most of his work was done for the Rockefellers. The wood his daughter buys might have a different criteria and of course. really premium, old growth slabs are very hard(and expensive) to come by.

Really? Was pretty clear in this post that he was talking about Mira. I also am not sure why that premium old growth stuff isn't at the same risk of cupping as the junk Mira is picking up these days. Might be due to the fact this junk wood Mira picking up is not intended to be sold to the Rockefellers.

[Jim Matthews]

If the table was a dog table,I'd be careful of it lifting 1 leg off the floor!!!!:) Why did I type that?

Ewww.
Funny -
but ewww.

[Robert Johnson]

Thanks for all the great advice! I started by planing both sides of an off cut that will become one of the legs. Here's a photo. I'm considering orienting the concave side up and leaving some of the rough wood in the center (as in the photo). I need to put a butterfly or three in that crack though before I can continue. I'm probably going to use some cherry that I have in the shop.

[Prashun Patel]

That's pretty. I like the idea of that rough river running through the center.

Think long and hard about butterflying. Aesthetics are to each his own, but my 2cents is that they become the focal point; I think it would be at odds with the gouge marks. Instead, consider just epoxy filling and/or keying it from the bottom.

[Sean Hughto]

I like butterflys. They don't have to be a focal point - you can use the same wood rather than a contrasting one, for example. I like to play with the shapes too, and in doing so, you can even make the grain blend some - and curved lines stand out less than straight ones on a natural surface.
http://farm6.staticflickr.com/5480/11051386476_1942053739_c.jpg

On this particular tabletop, the ambrosia marks and grain were so lively, that even the multiple contrasting butterflies don't become the focal point - at least o my eye.
http://farm8.staticflickr.com/7296/11051299895_a0311e867f_c.jpg

[Sean Hughto]

Here's another - this time a benck top in box elder - and with a cherry butterfly (top). Again, owing to the striking box elder grain and the inlays around the leg tenons, I don't think the butterfly is a focal point or destracting.

http://farm4.staticflickr.com/3628/3317604175_0ffa25cc4d_o.jpg

I guess this is all to say in reaction to Prashun's comment, that I think it all depends - like so much in woodworking (and life).

[Brian Holcombe]

I don't use butterfly's, that being said I think they look great when done tone on tone with a rustic piece.

I feel the same way in regard to all exposed joinery; use it when required and keep it subtle. Not always tone on tone, but I'm not into high contrast.

[Pat Barry]

That's pretty. I like the idea of that rough river running through the center.

Think long and hard about butterflying. Aesthetics are to each his own, but my 2cents is that they become the focal point; I think it would be at odds with the gouge marks. Instead, consider just epoxy filling and/or keying it from the bottom.

I'll be contrarian because I wouldn't want that rough surface to sit on if it were me. It seems not much more material to remove to eliminate it completely. Now, I suppose if you were going for a completely rustic look then at least sanding to eliminate any potential slivers (ouch) would be necessary. If you are set on the gouges being left behind :( , then at least fill the grooves in with something like clear epoxy so you can still see the effect but not feel it.

[Prashun Patel]

I shouldn't have made a blanket statement. Again, to each his own, but personally, butterflying a long split like that right on top of all that gouging will be an aesthetic challenge.

[Sean Hughto]

Prashun, it's okay if you don't like butterflies. I was just offering my personal tastes, not trying to say your taste or insight about how one might think about when or where to use them was wrong.

As far as this piece of wood, I can see why Robert likes the gouges. It reads like a River bank full of trees with the crack as the waterline and the gouges on the other side the reflections inthe water. Maybe there would be a way to add shapely butterflies that emulate more trees and their reflections? It would be easy to finish the tops of the inset pieces with a gouge to match or emulate the surrounding wood.

[Sean Hughto]

By the way, I love Robert Johnson!

http://www.youtube.com/watch?v=4up4VP8zjyc

[Jack Curtis]

By the way, I love Robert Johnson!

Great crowd to hang with. Now if you want to get down to it, try some of the chants and calls RJ probably listened to, if you haven't already.

[Scott T Smith]

Hi guys. I'd like to take a minute to share some knowledge that may be of benefit regarding wood movement.

There are actually four basic "rules of thumb" that I suggest when considering slab wood movement. The first (and most commonly known as others have alluded to), is that wood cups towards the bark. The easiest way to remember this is that the growth rings - when viewed from the ends of the slab, will try to straighten out as the wood dries.

This rule is fairly widely known, but the other rules are not.

The second rule is that sapwood is usually under tension as related to the heartwood of the log. When I mill a slab, if I remove the sapwood along one edge of the slab, but leave it in place along the other side, the slab will almost always crook towards the sapwood side of the slab as it dries. It is for this reason that many long boards that are quartersawn may have a crook in them, as frequently there may be a band of sapwood along one edge of the board and the tension in the sapwood causes wood movement.

Another poster mentioned that medieval tables often had one flat side exposed, but the bottom side of the table often had the sapwood left in place (envision a table made from 1/3 of a log with the flat side up). As the cut side of the slab dries, the shrinkage experienced during the drying process will try to cup it towards the milled side. By leaving the sapwood present on the bottom side of the slab (such as a half moon slab), the tension in the sapwood will counter the drying stresses on the opposite side and the slab will stay relatively flat. A few years back I milled a 40" wide red oak slab about 14" thick, and left the bottom side of the log intact on the slab. It dried relatively flat from side to side and end to end. I recently made this slab into a bench for my wife to use next to her horse barn. There was no significant wood movement on the milled side of the slab after several years of drying.

The third thing to keep in mind re slab movement is if your growth rings are not consistent from side to side in the slab, there will be inconsistent movement during drying. If you have a board that has the pith significantly offcentered, be prepared for some cupping near the pith but the remaining portion of the board to dry flat.

The fourth thing is that the amount of movement experienced by a slab during the drying process varies depending upon the distance the slab is from the pith. Slabs milled adjacent to the pith will usually develop a sharp cup right next to the pith. The slab almost forms a v-cup, instead of a gradually rounded cup. Slabs milled closer to the bark will have a more gently rounded cup from side to side. Slabs milled about half way in-between the pith and the bark usually have the lowest amount of movement along with the best figure.

Regards,

Scott

[Robert Johnson]

@Prashun, I agree that the dovetail wil interfere with the look of the gouged surface, so I'm going to insert the dovetails from the other side which is flat down the center. I'm also going to run a little glue into the crack, but I read a FWW article that said type 1 PVA (Titebond III) was stronger than epoxy in joints with gaps. Go figure.

@Sean, I really like your shapely butterflies! What an original idea. They fit right in on your piece with the wavy grain patterns; there's some contrast, but they don't become a focal point. BTW, I listen to Robert Johnson in the shop. Music in the shop is one of the many benefits of working with hand tools.

@Soctt, my slab includes the center of the tree on one surface, so it has the V-shaped cup exactly as you describe. I'd have to remove twice as much in thickness (much more in volume) to get rid of the gouges. This would reduce the thickness by nearly 1/2 inch. I think the tree is telling me to leave the gouges! Besides, the gouges were made by hand by my wife's Mennonite grandfather over 30 years ago, so I have to leave his signature on the piece.

[Robert Johnson]

Here are a couple of photos after I flattened both sides. I had to remove about 5/8 inch using a heavily cambered blade in an old Record jack plane. I smoothed it with a slightly cambered jack plane, then a smoother. It was a lot of work, but I like the result. Now I'm ready to dovetail the completely flat side, which will become the inside of one leg.

[Andrew Pitonyak]

Nice job.... nice looking slab too.

[Gary Muto]

Hi guys. I'd like to take a minute to share some knowledge that may be of benefit regarding wood movement.

There are actually four basic "rules of thumb" that I suggest when considering slab wood movement. The first (and most commonly known as others have alluded to), is that wood cups towards the bark. The easiest way to remember this is that the growth rings - when viewed from the ends of the slab, will try to straighten out as the wood dries.

This rule is fairly widely known, but the other rules are not.

The second rule is that sapwood is usually under tension as related to the heartwood of the log. When I mill a slab, if I remove the sapwood along one edge of the slab, but leave it in place along the other side, the slab will almost always crook towards the sapwood side of the slab as it dries. It is for this reason that many long boards that are quartersawn may have a crook in them, as frequently there may be a band of sapwood along one edge of the board and the tension in the sapwood causes wood movement.

Another poster mentioned that medieval tables often had one flat side exposed, but the bottom side of the table often had the sapwood left in place (envision a table made from 1/3 of a log with the flat side up). As the cut side of the slab dries, the shrinkage experienced during the drying process will try to cup it towards the milled side. By leaving the sapwood present on the bottom side of the slab (such as a half moon slab), the tension in the sapwood will counter the drying stresses on the opposite side and the slab will stay relatively flat. A few years back I milled a 40" wide red oak slab about 14" thick, and left the bottom side of the log intact on the slab. It dried relatively flat from side to side and end to end. I recently made this slab into a bench for my wife to use next to her horse barn. There was no significant wood movement on the milled side of the slab after several years of drying.

The third thing to keep in mind re slab movement is if your growth rings are not consistent from side to side in the slab, there will be inconsistent movement during drying. If you have a board that has the pith significantly offcentered, be prepared for some cupping near the pith but the remaining portion of the board to dry flat.

The fourth thing is that the amount of movement experienced by a slab during the drying process varies depending upon the distance the slab is from the pith. Slabs milled adjacent to the pith will usually develop a sharp cup right next to the pith. The slab almost forms a v-cup, instead of a gradually rounded cup. Slabs milled closer to the bark will have a more gently rounded cup from side to side. Slabs milled about half way in-between the pith and the bark usually have the lowest amount of movement along with the best figure.

Regards,

Scott

Scott,

What a wealth of information. Thanks for sharing!