Help me settle a bet. Why does fiber paper curl?

[DrPhil]

I am hoping that someone out there can settle a disagreement that a buddy and I have. After setting up my new enlarger (8x10 durst) a buddy and I started looking at a few prints on fiber paper. I have always thought that fiber prints curled because the emulsion dried faster than the paper base. My buddy disagrees. He believes that fiber paper curls because of the grain. Which of us is right? Or are we both wrong? Thanks

 

[Thomassauerwein]

I don't know the tecknical but I've found the more the paper obsorbs the more it roles up during the drying process. Some papers just seem to swell more than others.

 

[Donald Miller]

I don't know that this is accurate but I have a thought that this has to do with the relative rates of absorbtion. expansion, and contraction. What I mean by this is that the gelatin emulsion does not absorb as much water as the paper support because the paper support has a greater capacity for water by it's nature and it is thicker. The gelatin base is separated from the paper by a baryta layer which is a clay substance. The baryta layer serves two functions in the paper. The first is it provides the color of the base and secondly it prevents the absorbtion of the gelatin emulsion into the paper fibers at manufacture. The greater paper thickness just amplifies the condition of disparity even more. This relative capacity means that something has to give if water is going to be added...the only way that this water can be contained is by expansion of the paper base and the gelatin. When we dry the paper because of the relative differing amounts of water contained, the gelatin layer dries more rapidly and this causes contraction of that surface before the paper base dries and contracts. I don't think that it has much to do with the grain of the paper since the paper is not something that exhibits grain in the way that wood does for instance. If it were a matter of grain then the curling would be predictable and more inclined to one dimension then the other. If there is a greater preponderance of curling of one dimension over the other then this is explanable by the relative mass of paper along a given dimension available to contain water, hence to expand and to contract.

 

[Jorge Oliveira]

I don't really know why, but for sure it's not related to grain (if you mean picture grain).

Grain is just microscopic silver pieces crumpled together, loosely held together by the gelatin.

 

[Ole]

Well, you're wrong

Paper does have grain, in the sense that wood has grain. There is a slight but significant difference in the paper structure along and across the fiber direction. And when it swells, it swells more in one direction than the other. Then when it dries again, it shrinks more in one direction, causing it to curl.
Over time the differences settle out, which is why most paper eventually goes flat(tish).

Putting gelatin on this makes it even worse, as the drying rates of gelatin and paper are very different. Gelatin dries faster, causing paper curl again. The direction of the grain in the paper determines how it curls.

But gelatin has an equilibrium water content that is higher than that of paper. So at first the gelatin is drier, causing it to curl emulsion in. Then the paper dries, curling the other way. If there were any water spots on either side of the paper, you get "bulges".

More precisely the paper absorbs a lot of water without swelling much, in the intersices between the fiber. Swelling takes a bit longer. The gelatin absorbs water ONLY through swelling. So that water takes longer to absorb. and longer to release.

But the fibers in the paper will also absorb water and swell - and that, since the fibers are surrounded by water - at least in the early stages of drying - takes even longer to dry out...

So the curl is caused by the different swelling characteristics of paper and gelatin, and controlled by the direction of the grain of the paper.

Note that most photographic paper prefers to curl along the long side, into a tall cylinder. That's on purpose, paper is almost always made so that it tears more easily along the vertical than the slightly more vulnerable right across.
The only exceptions to this I know of are some small odd sizes which can go in any direction. I think Fotokemika Zagreb makes them from scrap...

 

[noseoil]

Ole is correct. There are two types of "grain" which may be confusing the issue. Silver grain is metallic, so it does not swell. Paper does, in fact, have grain very much like wood. When it is manufactured, the pulp is applied in a layer of slurry and rolled out into sheets, dried, coated with clay and finally coated with emulsion. Paper is made from wood pulp fibers (unless it is cotton or some other fiber).

As Ole has said, the grain of the "wood pulp" swells when it absorbs water. It swells more across the grain than in length, exactly as wood swells. When it shrinks back after exposure to water, it has changed its internal arrangement slightly. Due to the gelatin and clay, the pulp changes size more rapidly than the the other surface. This creates a tension on the gelatin side and a compression on the paper backing side. End result is the curling. With the application of heat and pressure, the paper becomes stable again and can be made flat.

Wood behaves in the same way. A timber which is dried too fast shows "checking" on the outside faces (cracks). This is the result of the surface drying at a faster rate than the center. The cracks start at the surface and extend into the center as drying continues. At the center the wood is still wet, on the surface it is drying, the surface splits open to maintain an equlibrium within the timber.

Wood expands more across the grain than in length. This is why a door which swells in rainy weather normally sticks at the lock side but not at the top or bottom. When we hang doors here in Tucson, an allowance is made if the weather is dry (8% relative humidity) compared to the rainy season (90% humidity). When you lay planks for decking, the growth rings should always curve toward the surface you walk on (center of the curve faces up). When humidity changes and the wood swells and changes shape, the center of the plank lifts. If the deck is laid differently, the edges cup upward and people tend to trip more.

So, how much money is involved in the bet?

 

[DrPhil]

No money involved. Just the I told you so rights. My buddy deserves some money or a cold beer though as he helped move a 600lb enlarger up stairs.

It sounds like we were both somewhat correct. The paper's grain influences the alignment of the curl while the different rates of drying (a result of different absorbtion rates) cause it to always curl towards the emulsion.

Thanks for the help.