i found this from another site:
The cells in old comb often appear smaller than they really are... The cell rims are thicker in axial depth and have a higher wax to pollen ratio than newer comb. The mouths of the cellrim are often more truly circular in old comb whereas in new comb the shape is a hexagon with rounded corners.
I once, (1989 or 90), came accross some combs that had been in continuous use for 22 years, the bees were healthy, fairly dark coloured, nice temper. The combs were solidly black, but smelled sweet. I persuaded the owner to let me cut a chunk from the centre of the most central brood comb, as luck would have it there were only eggs and young larva in that position. I used a very sharp pocket knife, but the cutting was very difficult with a great deal of tearing rather than clean cutting. When I got the sample back to my factory I put it in the fridge alongside the milk, temperature 3 or 4 deg C, I cut many cross sections using various blades, (I had to re-cool it several times as well), with varying degrees of success... The mid rib was 6mm thick but the cell walls were no different to fresh cells although the bottoms of the cells were 'bullet shaped'. You could see where the bees had chewed back the cocoons and then sealed over the exposed laminations. I think the bees repeat this process at about 3 or 4 year intervals. I have given old comb to several swarms (I prefer to use foundation but sometimes needs must...) The results were about a 50/50 split with some combs used straight away and others chewed up and fresh walls added.
We now know that we have been providing our bees with oversize foundation for about a century. I feel the so called smaller bees are merely natures attempt to redress the balance. And that if comb were to be kept in use for a long time a stable situation would occur whereby the cell walls would be chewed down and replaced on a regular basis.