Every sheet of paper to be marbled must be coated with alum.  It acts as a color binder (or mordant), fixing the colors to the paper permanently.  If alum is not applied to the paper before marbling, most of the color will rinse off.  The alum should be pure aluminum sulphate; do not use alum containing potassium, ammonium, or anything else, since these extra chemicals may harm your paper.  Spice-rack alum does not work well.

Now with a small sponge, wipe some of the alum solution across one side of each paper.  Don't miss any areas of the sheet, or the color will not stick on the patch that was missed.  It is usually best to wipe the whole sheet thoroughly in one direction, then go back across it the other way to make sure it has been completely covered.  The paper should not be dripping wet though, or it will not dry out very smooth — just get it well dampened.  It is okay to put your fingertips on the paper as you are wiping it.  Lay out each paper flat to dry.

If you are working on a large number of sheets, especially if they are large sheets of paper, the above method will be too slow.  In this case you should apply the alum by the soaking method:

            Put one-third cup of alum into one gallon of water (or you can cut this recipe in half).
            Stir for 2 or 3 minutes, till the alum is dissolved.  Pour this mixture into a large marbling tray. 
            Lay a sheet of paper on the surface of this alum water.  Press one end of the paper to the bottom of the tray.
            Using a skimmer stick (a thin strip of wood about the width of your tray), push the rest of the paper
            to the bottom of the tray by running it across the sheet lengthways. 
            Do as many more sheets as you wish on top of this sheet, making sure each one is totally soaked.
            Leave them in the water a few minutes, then pick the whole stack up, and set it on a slanted board to drain.
            Peel each sheet off the stack and hang it individually to dry, preferably over a rod or tube. 
            Rinse your tray out very well, since excess alum can contaminate any carrageenan you may put in it later.  

When your sheets are all dry, stack them up and put them under something flat such as a board or book, and then stack some more weight on them, for several hours or overnight.  The papers will need to be as flat as possible before marbling; it's very hard to marble well on wrinkly paper.

Alum is not a dangerous chemical, it is used for water purification and in some food processing.  Still, it should be kept out of reach of children.  It tends to dry out the skin, so wash your hands after using it, or wear gloves.  Avoid inhaling the dry alum dust, which can irritate your nose.

The alum process can be done a day or two ahead of time.  You can treat many sheets at a time so they will be ready when you want them.  However, alum-treated papers which are exposed to the air for more than a few days will oxidize and start to lose their alum, and will need to be redone.  Leftover alum-water mixture will also go bad, and can only be used for one or two days.  So only mix up as much as you will need each day.

If you are marbling on fabric, the alum mixture does not need to be quite as strong as on paper, since cloth will soak up and hold much more alum than paper will.  To apply alum to fabric, it is easiest to soak it in the alum water rather than applying it with a sponge.  Mix one-fourth cup of alum in one gallon tap water, or one-eighth to a half gallon water.  Pour it into a large bowl, fold your fabric, and soak it in the alum water for 10 minutes or so.  Lightly squeeze the cloth to drain it, but don’t wring it out or it will wrinkle too much.  Hang the cloth indoors till it is dry.  Iron the cloth smooth.  It is now ready for marbling.

Occasionally you may hear some marblers claim that they get good results without using alum.  But if they were to compare two papers done both with and without alum, they would find out:  you will always get better results with alum!!   The colors will be brighter, and lines sharper and clearer.