How to fix cracks in a Fritz Hansen Ant chair?  

I think...
You would need some serious tools to be able to glue that back properly. Shame since the 3105 is harder to come by.
I never sit in my Jacobsen plywood chairs.

My suggestion
use good wood glue and plenty of tight pinch clamps. Once they're totally glued, then lightly sand them, but you'll probably have to match the color of cracked wood.
They must've suffered some serious misuse to crack like that.
Good luck, tho.

A fine syringe
A fine syringe with wood glue will fix those cracks. Squirt it in and clamp it tightly. The problem is that whatever caused them to crack in the first place - dryness, overexerting the wood, an inherent weak spot - may still manifest itself elsewhere.

I was thinking of removing...
I was thinking of removing patches of all the layers that are cracked, and make them of increasing and thus overlapping sizes so that you do have a significant glue area/surface.
I am affraid that it will crack again when just glue those cracks because the surface amount to glue is relatively small.
André

buckle
the ant chairs and all the laminated chairs by Jacobsen are prone to weak backs and break if the load or person is to great.
There is a small metal bracket much like an over sized Y the strengthens the back after glueing.
The re-upholster
It is why you see so many Jacobsen chairs upholstered in poul smith fabrics doing the rounds. They have been repaired and the upholstery obviously covers all that nicely.

And that's why...
I never sit in mine.

fixing laminate proposal?
Could the link below be a proposal to fix the crack?
Note that the photo's are in reversed order. The fixing sequence are the number at the middle top of each picture.
The nice thing about this fix is that you will see only at the back that it has been patched/fixed, the front stays the same.
Thnx
André
http://www.flickr.com/photos/founditems/sets/72157617998416164/

Andre wrote:
"I am afraid that it will crack again when [I] just glue those cracks because the surface amount [of] glue is relatively small."
You're right, I'm afraid. The largest (i.e.,bottom) patch would have to be at least 4" wide, I think, in order for each layer to have sufficient glue area. But it's a nice strategy, theoretically. The difficulty is in accurately removing the various layers, which would be difficult enough on perfectly flat material. In this chair, the affected area is curved and wavy.
(You've drawn four layers; all plywood has an odd number of plies (3, 5, 7 etc.)
You should be able to introduce glue into the breaks. Mask carefully on either side of the top surface crack so glue doesn't stick to the surface, and devise clamping blocks that match any curvature in the area that needs to be clamped. (One method is to start with flat blocks, and build up the curved contours with layers of masking tape.) Wax the finished blocks to avoid gluing them to the chair. Do a test clamp-up, with no glue, to assure that there is even pressure over the glued area. If you're lucky and careful, no sanding will be necessary on the top of the chair.
Then, don't use the chair for seating. Seems like a lot of trouble for little gain.

the Jacobsen chairs have 9...
the Jacobsen chairs have 9 (really thin) layers if I'm not mistaken. You'd also loose the reinforcement of the 2 cotton layers...
But It Ain't Over 'Til It's Over, do what you can!

"Seems like a lot of trouble...
"Seems like a lot of trouble for little gain." - SDR
My sentiments as well. These chairs are common enough to get at a reasonable price. Especially compared to the potential agony of repairing yours. My personal approach would to use it for parts.
It's a shame Jacobsens plywood chairs have such a fragile quality. I'm always giving the chair too much attention, fearing a snap...and I only weight 180lbs. Lovely chair no doubt!

Although I can't give any...
Although I can't give any advice for repairing these chairs, I'm must say the chairs are not that fragile in my opinion.
Here in Scandinavia, they are still everywhere, in homes, restaurants, auditoriums, etc. (much like the Eaemes stackers in the US) Only once have I heard of a chairs back actually breaking and that was from a pretty big guy, leaning back heavily onto the back rest.
Unless it's an extremely early example, with a high value, I wouldn't be afraid to sit in one...
Something I HAVE seen, is people falling off the three-legged version...

pros and cons of repair
if the chairs are going to take you days and many hours of work to repair you need to balance this against the minimal value they will have afterwards and balance this against the cost of getting some good ones.
A repaired chair will have very little retail value against a good one so unless you are doing the repair for the challenge of your skills or the love of woodwork and understanding stress fractures, then put them to rest in the high heaven of dead designs.
YES in Denmark many restaurants and universities have these chairs but they are well aware of their issues and therefore change at regular intervals.
That is why there is currently 2500 grandprix chairs being sold all with the link bracket. They have badly worn sides from stacking and backs that would need the scrutiny of a brain surgeon so you get a good deal and not a lame horse.
Personally I stay well clear of Jacobsen chairs as I feel that they are over rated and "common as muck" unless it is a set I know has come from a private home where care and love has been lavished on them.
Still thats only my oppinion and I still find that the chair works a sculpture that should be enjoyed for its simplicity and looks and rarely used.

I do have to agree that...
I do have to agree that these tiny chairs are are a great design. I do have a SuperElipse table and I bought 4 Butterfly chairs made in '99 and they are in great shape and do not have any cracks. However these very old musquitos would be very nice when we do have company and the kids can sit on them so that I can seat at least 6 or may be even 8 (4 Butterflies and 4 Musquitos) around this great looking table. Normally I do have 4 chairs around the Elipse and there is more than plenty of room. However, it would be nice so that an adult could sit on these chair without the risk of breaking them. So my idea is to fix them also to gain woodworking experience and I do like to exchange ideas with other people around the globe. When I fixed them I fill and sand them so they will have a flat surface so you would not see the wood grain and the area that has been fixed and then give them the same color as the Butterflies. That is so far my plan. To spent some time doing this is fine by the way. It is not a commercial project but a hobby instead and have something unique furniture in my house that can't be bought @ Ikea....
I have been devorced now for 4 years and I can do what I like in my house and it will become a nice place at some time. There is nobody that tells me what not to do.
André
(Netherlands)

Looking
at this particular damage, I would have to say that the chair, once repaired, will never be as reliable as an undamaged one -- short of adding an additional layer of veneer over the damaged area, on both faces of the chair. And that could never be disguised.
So, if I could "tell you what to do," I would say to forget this chair, and replace it -- if you wish to subject it to normal use. But if you enjoy repair for its own sake, do proceed -- and please let us know how you do.