I got the idea for this week's experiment while working on the rocks we collected recently. When we got home last weekend, we unloaded the 750 pounds of quartz crystals from our Arkansas trip onto the back patio. Then I left for Philadelphia. Today, I went out to unpack some of the boxes. We had wrapped the best crystals in newspaper, and all the newspaper that was exposed to sunlight had turned yellow. Cool! A chemical color change that happens fairly quickly, and is caused by exposure to light! I have seen the same thing many times before, but this time it hit me as a potential experiment.

To try this, you will need:

- newspaper
- a sunny spot that will not be disturbed
- several objects
- a pencil or pen

Place a sheet of newspaper on a flat surface, where it will get plenty of sunlight. Place a weight on each corner, to be sure it stays in place. Pick a spot and write the word "control" on the paper. Then place an object over the word. Use something like a saucer, a coffee cup, or a small book. Be sure the object is opaque, so no light can get through to the paper underneath. Pick another place on the newspaper and write the word "observation." Put a second object over that word.

Why did we write the word "control" under the object? In a science experiment, you use a control to help be sure that you are seeing what you think you are. We are expecting the paper that is exposed to light to change yellow. If the paper underneath the control object changes color too, then we would know that the color change was not due to light.

After an hour, check to see if something has happened. Do not move the control object. We don't want any light to hit the paper underneath. Instead, move the object with the word "observation" under it. If the sunlight was bright, you may already see a difference. Under the object, the paper should not have changed, but the paper around it may have a yellowish color. Put the object back in place. Pick a spot on the sheet of paper and write "one hour." Cover that spot with another object.

Wait three more hours, and repeat the process. Again, don't move the control or the one hour object. Just move the one labeled observation. Notice any difference in color. Pick another spot and write "three hours," and cover it with another object.

Wait until the next day, and repeat the process again, labeling the new spot with "24 hours." Wait another day and repeat, labeling the new spot "48 hours." You can keep the experiment going as long as you have room for more objects. When you are done, remove all the objects and compare the spots that were covered with the exposed paper. The control spot will still be its original color, but the longer the paper is exposed to sunlight; the yellower it becomes.

OK, so what makes the paper turn yellow? Paper is made from wood. Wood is made up mostly of cellulose and lignin. For paper, cellulose is the "good stuff," while lignin is not. Cellulose is made up of long strands, which matt together to give us a smooth, flexible writing surface.

Lignin makes paper dark colored and more rigid. That is great for making cardboard boxes and paper bags for grocery stores, but not for books, printers, etc. Good quality paper has had most of the lignin removed, but newspapers are printed on cheaper paper with the lignin left in. Chemical treatments can bleach the paper to take away the dark color, and make is less rigid, but over time, lignin breaks down, causing the paper to turn yellow and become brittle. As with most chemical reactions, adding energy will speed up the process. Light energy and heat energy both cause the lignin to break down faster. You get a similar reaction when you toast a marshmallow. The heat from the fire breaks down the sugar molecules, producing a brown color. Great for marshmallows, especially when combined with graham crackers and chocolate. Not so great for paper, with documents turning dark and brittle.

There are ways to slow down the yellowing. Natural acids from the wood, along with acids which may be used in the manufacturing process also speed up the process of yellowing. By treating the paper with an alkaline solution, you can neutralize the acids, giving your paper a much longer life. You can buy acid free paper, buy special chemicals to do neutralize the acids, or you can soak your newspaper clippings in a solution made by dissolving two tablespoons of Milk of Magnesia in one liter of club soda. Let the solution sit for 8 hours before using. Be sure to test the ink on your paper first, to be sure it will not smear or run. Then soak the paper for an hour, and CAREFULLY remove and let dry.

This experiment brought back memories of when I took a job as Curator of Education and Collections at a museum in Florida. While inspecting the collections, I was pleased to see row after row of acid free storage boxes, specially made to protect the artifacts inside from the destructive acids in paper. I was not so joyful when I discovered that all the artifacts inside those expensive, acid free boxes were wrapped in newspapers! They knew they were supposed to have acid free boxes, but did not know the science to understand why. That day they learned a lot of new science!