I found the following recipe online from an 1868 cookbook -- do you think it would work?

PRESERVED CITRON

Pare off the outer skin, cut into halves, remove the seeds,
then divide each half into a number of smaller pieces. Put
them in a stone jar, add a half-cup of salt to every five
pounds of citron. Cover with cold water, and stand aside
for five hours ; then drain, and cover with fresh, cold
water. Soak two hours, changing the water three or four
times. Dissolve a teaspoonful of powdered alum in two
quarts of boiling water, add the citron, and bring to boiling
point. Drain. Make a syrup from two and a half
pounds of granulated sugar and one and a half quarts of
boiling water, boil and skim. When perfectly clear, put in
the citron and simmer gently until you can pierce it with a
straw. When tender, lift the pieces carefully with a skimmer,
place them on a large plate, and stand in the sun one
or two hours to harden. Peel the yellow rind from one
large lemon, add it to the syrup, then add the juice of two
lemons, and a small piece of green ginger-root cut in thin
slices. Boil gently for ten minutes, and stand aside until
wanted. When the citron has hardened, put it cold into
the jars, bring the syrup again to a boil, and strain it over
the citron.
Watermelon rind and pumpkin may be preserved in
the same manner.

One thing about watermelons. To know when they are really ripe, look at the place where their stems attach to the vines. Just opposite in the same locaton as the vine and stem is a thin dying tendril. This tendril will usually show a tan coloration as they die off. Picking melons sooner will give a less sweet melon. This year, I grew a seedless type and they require a pollenator type to be planted close by just for the need of cross pollination. The pollenator is a seeded watermelon, and these have dark green thin rinds/skins, compared the seedless, which are a lighter green color. I must say that I was very impressed with the seedless as it was as sweet in the edges as it was in the middle. The 10 seeds of seedless watermelons I got were all started indoors in early April. The cost of just 10 seeds is about $20! There is basically just one single viable seed in a single seedless melon. These are nearly the same size as a regular watermelon seed, but have a brownish color. The rest of the seeds in seedless melons are very thin, white and very tender like a young cucumber.
Alum used in pickle recipes today offers some risks. It was ONLY used to give pickles and acidic products a little 'pucker'. Its aluminium based, and has not been recommended for any home canning. The ONLY place I use a very small amount is in sour full strength vinegar pickles where that necesary 'pucker' is desired. Alum offers nothing but a dry, puckery taste, and will not improve texture in any way.

Sure it would work. Process as for any other preserve for 10 minutes.

Skip the alum. It only works for fermented pickles. And even if it did work, what's the use of including a crisping agent in a recipe where you're supposed to cook until the product is tender enough to pierce with a straw?

Carol

Alum is NOT a 'crisping agent' and never was intended for that purpose!! Its only use was to enhance the dry puckery taste of a very strong vinegar sour pickle. Today, alum is not used in any recipes much anymore due to possible health issues it can create. Think of it as adding ground up aluminium to your canning!! If you want to retain crispness or firmness, you have the option of adding calcium chloride to 'buffer' the softening effects vinegar has on foods. Fermented pickles are not using alum either. Someday, it may end up completely off the ingredient lists for home canning. Even for that, pickling lime can still be used, but requires a lot of soaking, washing, scrubbing, and rinsing prior to filling jars with the vegetables. I have a lot of pickling lime here, that will eventually end up as garden pH adjuster .

Ken, you are mistaken.

Please note this quote from Utah's extension service:

"If you choose to use firming agents, alum (aluminum potassium sulfate) may be used to firm fermented pickles, but has little crispness effect on quick-process pickles. Alum will increase firmness when used at levels up to 1/4 teaspoon per pint. Addition of greater than 1/4 teaspoon alum per pint will decrease firmness."

Or how about the USDA Complete Guide to Home Canning:

"Alum may be safely used to firm fermented pickles."

Alum is definitely a crisping agent and is called for in many traditional fermented pickle recipes, including quite a number of sweet pickles and products like maraschino cherries.

It is no longer necessary in many recipes as its use has been supplanted by other products, but I could find hundreds of reputable sources which list it as a firming agent.

And yes, it is an astringent in such things as styptic pencils, but one use doesn't preclude the other.

Carol

Well, LOL, I didn't mean to start an alum controversy, but now I'm rather curious about the disparate opinions about it. I've used it in various recipes over the years, mostly, I guess, because they called for it. I know it must be one of those "generally recognized as safe" type of food additives, because it's STILL sold -- I ran out and bought some at Kroger just a few weeks ago.

I'm not too worried about the health effects, or lack thereof, of alum, because hey, there are a WHOLE lot of things more likely to kill me, IMO, than a little tiny bit of alum. I'm not trying to make fun of the possible effects of aluminum consumption on Alzheimers or other disorders, but, if you saw the traffic I drive in to get to work, with all of the nutso drivers on the road, you too would realize that consuming small amounts of aluminum once in a while aren't the biggest risks to my life that I face!

I would like to try Calcium chloride for some of my pickles, but I can't find it anywhere in food grade.

So, what is the scoop or consensus on alum -- forget it totally, use it only in fermented pickles as stated above, or use it when the recipe calls for it and don't worry about it?

For all the reasons stated, I still would not introduce aluminium or any of its various derivitaves in pickling. A pickle is a thing where vinegar, salt, acid and such work on the vegetable to prevent spoilage. as a 'firmness' agent, its not even mentioned very much in most recent recipe books. Commercial pickling also has not used it for many years now. It might be fine for a 'styptic pencil' to stop a bleeding cut on the face, but thats agout it.. In the same link, they say not to use aluminium vessels for pickling..

My links contradict yours, but do state that it 'could' be used, however they do not recommend it, and further state that pickling lime would be safer to use IF you needed help in keeping things crisp.

http://www.straightdope.com/columns/read/1397/what-is-alum-and-will-it-make-someones-mouth-shrink-like-in-the-cartoons

Here is a link that might be useful: No alum used or necessary

If you want crisp pickles, pickle the rinds. No need for additives of any sort. You don't even have to pickle them immediately after picking. You can sit the rinds on the counter for hours, pickle them, and they'll still be crisp. (I understand cucumbers should be pickled immediately after picking for optimal crispness.) This is my experience with a Jubilee watermelon rind, anyhow. I guess Red-seeded Citron watermelon rinds might be different. I know this thread isn't about the rinds, though. So, I'm just interjecting (I'm not meaning to steer discussion off-course).

Alum is totally still used in commercial store bought pickles! And yes, it is used to make crunchy pickles. Not all of them do, but the ones advertised as being extra crunchy do. Whether it is harmful or not i can't say, but commercial pickles don't taste as good to me as homemade ones. They have to be crunchy though or i wont eat them. A natural alternative that also has better flavor is to use the old time pickle recipes that call for grape vine leaves. We have a concord grape vine and the leaves make excellent crunchy pickles.