Clock case repair, bowed back

[rmp]

Hello

I am restoring a clock case, which is Mahogany. The back of the case has bowed in a concave fashion and will not be flat against the wall. There were originally three battens accross the back of the case, but these have been cut and are not re-useable.

My plan is to replace the battens, and I have tried to cut a concave hollow in the battens to accomodate the bowed back of the case. I used pieces of oak, but the result was not very satisfactory.

My questions are:

1. What is the best way to transfer the shape of the case to the battens to ensure a nice snug fit?

2. What is the best way to cut the battens? I have tried using a spokeshave, and cutting lines with a tenon saw and chiseling out, but not a great outcome.

3. I wondered if it might be easier to use a softwood, then add oak fillets to the ends for aesthetic purposes. However, would the differing expansions rates of the two woods cause even more problems in the future?

The width of the case is about 300mm and the battens are about 60mm by 10mm deep.

I would be very grateful for any advice.

Thank you,

Richard

 

[Orraloon]

Kind of hard to advise without some pics of the problem. The back being bowed sounds like a problem that needs to be addressed as it may cause further damage to the case. It may be forcing the sides out. Is the clock antique or valuable.
Regards
John

 

[mrpercysnodgrass]

I would have thought taking the back off and straightening it will be the way to go. As John said some photos will help... of the inside as well as the back.

 

[rmp]

As far as I can tell, the sides do not appear to be forced out. It's about 100 years old and in terms of value, certainly not something I'd want to risk damaging. Regarding straightening the back, how would you go about that?

Anyhow I have attached some pictures as requested.

 

[profchris]

What you seem to have there is the board which makes up the back shrinking across its width. Because of the way it was cut, the back has shrunk more than the inside, so it has cupped towards the wall. This is just what wood does as the humidity changes - if you put the clock in a damper environment it might straighten out, but would move again if you moved it back.

Even if you straightened it out by humidifying it, then added battens, I reckon it would still cup nearly as much as it dried out again.

If you want to replace the battens for cosmetic reasons, matching the curve is just a matter of going very slowly and carefully. Get the profile close, then remove tiny amounts and offer it up to the board again. If you want as perfect a fit as possible, you could try getting it close with spokeshave/chisel/etc, then holding it against the back where it is to fit and flossing the joint with a strip of sandpaper (Sellotape on the back to help stop is tearing). It will take a long time, hours rather than minutes, to get all three battens to a really good fit. Don't forget that the curve in the back is not uniform, so each batten needs to be fitted individually to its location.

 

[Droogs]

if you don't want to play about with the back then get yourself a profile gauge, they come in various sizes or even make one with bamboo skewers and a couple of strips of mdf

examples here https://www.google.com/search?q=profile ... 00&bih=836

hth

 

[rmp]

Thanks for the help.

Do you think using a softwood wood create more problems due to differing rates of expansion? It would be much easier than using hardwood (having checked, the original battens seem to be softwood).

 

[Orraloon]

The back is going to keep moving as it is sealed with shellac or varnish on the inside while the other is open to the atmosphere. Being one wide board is another contributing factor. Central heating likely adds to the problem. Rather than add batons (that really I can't see being a help) perhaps leave it in a spare room without heating for a few months. If it settles down a bit then get a coat of varnish or whatever it is on the back . Unlikely to ever be dead flat again but that would be my first approach.
Regards
John

 

[mrpercysnodgrass]

First off Richard, it looks very much like a walnut case to me with the back being solid walnut, In thirty five years of restoring I can't think of a long case that has not had either an oak or pine back. Also the inside of the back has been varnished or polished in some way (another thing i have never seen before) which is probably where the problem lies. My first attempt would be to strip the finish off the inside and leave it for a while to see if the board straightens out all by itself. The only way to permanently straighten the back is quite invasive and would involve routing out channels along the grain from top to bottom then glueing in batons slightly smaller than the width of the channel.
The bow does not look that bad to me. Does it affect the way the case sits against the wall? As it would butt up against the skirting at the bottom the rest of the back would be the thickness of the skirting away from the wall and the baton that was originally near the top of the case would be a spacer to bridge this gap where it would be screwed to the wall and not necessarily there to straighten the back!

 

[rmp]

Thanks for the replies. The case could well be walnut as these clocks were made in walnut from time to time. The back is polished as the full length door is glass so it is visible. The clock isn't floor standing, it is hung on the wall.

Unfortunately the bowing means the clock doesn't lie flat to the wall. The battens do need to go back in some form as (a) they are how the clock was originally made, and I want to keep it as authentic as possible, and (b) the movement screws through the back into the battens.

It's been in a cold garage for a while, so I don't think the bowing is going to go down. I really don't fancy stripping the back or cutting into it, so I really can't think of an alternative to the curved batten approach....

 

[profchris]

If you think it's stopped moving you could just plane the back flat and then attach flat battens. Rather easier than matching curves ...

 

[mrpercysnodgrass]

Is it something like this one below? If so I think Profchris has the easy answer.

 

[Paul200]

If you think it's stopped moving you could just plane the back flat and then attach flat battens. Rather easier than matching curves ...

Darn it! I was going to suggest that! Would have been the first useful comment I've made on here and you beat me to it. #-o

 

[MusicMan]

Personally I wouldn't strip the inside of the back; that polish may be original and it looks good. Nor would I plane it flat, as (a) that is taking away original wood and (b) it may move in future. Indeed, if it has come out of a cold (damp?) garage into central heating, it is likely to move.

Likewise matching the curve may not be permanent, though you could scribe it using a pencil laid flat against the back, giving an offset to a batten.

However I would simply fix the flat battens in the centre of the back and make spacers that match the battens to fill in the gaps just at the sides. And I would only fix the battens near the centres; say with two screws about 10 cm apart. The side spacers would be glued to the battens but not to the back. Then it will look good and the wood can do its thing without splitting.

A coat of suitable varnish to seal the back would improve stability.

Keith

 

[Sgian Dubh]

If the panel is irredeemably cupped, i.e., the cupping is a result of permanent set and won't flatten naturally through changes in RH, and if you're not too concerned about modifying the back panel, one, two, or three longitudinal partial depth saw cuts would weaken the panel thus making it easy to mash it down on a flat surface to take out most, or even all of the cupping. You could attach bearers to maintain flatness, and/or plane up fillets to glue into the saw kerfs to hold the flattened panel in its new shape.

A track saw or similar would be one option for making the cuts, as would a router, side fence (or straight batten), and a small diameter straight cutter.

I have effected flattening of single leaf old and antique table tops that were hopelessly cupped through long term moisture cycling resulting in permanent set. In your case, moisture cycling being the cause of such cupping is much less likely than in table tops, but something similar might be a possibility. Slainte.

 

[rmp]

Thanks for all the advice. Interesting to hear such a variety of possible approaches. For the sake of completeness, I thought I'd reply with what I did.

I didn't fancy altering the existing clock too much, as from a conservation point of view I wanted to be as least invasive as possible. I didn't put battens across the entire back, but made two short pieces cut to shape either side. Thus either side is flat to the wall, and since the battens are not full width, hopefully problems with expansion/contraction will be minimised.

 

[deema]

I have flattened boards like this before. It doesn’t always work, but usually the bow is significantly reduced.
1. Remove all finish off all sides
2. Wipe a very damp cloth over the side that is raised, ie concave. You want the wood wet, but not soaked / standing water
3. Place the panel between a number of cowels and clamp, but not too hard, you don’t want to break the panel. Leave the panel out of sunlight place preferably in the room where it’s to be used so that air can circulate all around it.
4. Each day wipe the concave side of the panel with a very damp cloth and try to tighten the clamps a little more. The pressure on the clamps will relax as the wood straightens.
5. Once you have the wood flat by tightening the clamps, keep wiping with a damp cloth until you can unclamped without any real spring back, or if it’s a thin panel until the wood on the opposite surface starts to have darker areas were the water is soaking through. After checking the wood with unclamping, re clamp the wood again.

Now leave for about a month clamped up to dry out again.
The panel will either be flat or have the bow significantly reduced.
Either leave all surfaces unfinished, or finish all surfaces exactly the same.

Depending on wood species, the quicker method is to place the panel in a plastic bag (wall paper stripper outlet shoved in one end, small hole in bag the other, small supports top and bottom to keep plastic bag off the wood) and steam it for 1hr if it’s less than an inch thick, place in cowels to flatten and dry for a month.