Pickling lime is calcium hydroxide. It’s also called “food-grade lime”, because in making the calcium hydroxide the processors make sure that the process remains pure and doesn’t introduce anything untoward (e.g. it’s not done in rusty old bins.)
Pickling lime helps to improve the firmness of pickles by introducing calcium that reinforces the pectin in the vegetable being pickled. In using it, a vegetable such as cucumber is soaked first in water mixed with the pickling lime, for up to a day, then the food to be pickled is rinsed thoroughly — at least 3 times — before the actual pickling process begins.
Because pickling lime is alkaline, you have to get rid of it all. People haven’t always rinsed thoroughly enough, leaving some alkalinity and thus weakening the required acidity of the pickling batch. On account of this, cases of botulism have been recorded, and for that reason many reputable home canning authorities are reluctant to advocate its use anymore. Some swear by using grape leaves instead to help with crunchiness instead of using lime, most authorities say if you use fresh cucumbers and trim the blossom ends off properly, that grape leaves add no enhancement.
In no event substitute any industrial lime, whether agricultural or lumberyard lime, for pickling lime: lime may contain unsafe contaminants unless it is certified food-grade pickling lime.
When purchasing, also be cautious if you see “lime powder”. It may be lime (as in the fruit) flavoured powder, which is completely different. While it would not be unsafe to soak your produce in it, it may cause everyone at the church picnic to pucker up as they never have before.
Pickling lime usage by reputable home canning sources
The USDA (United States Department of Agricultural) Complete Guide to Home Canning says,
“The calcium in lime definitely improves pickle firmness. Food-grade lime may be used as a limewater solution for soaking fresh cucumbers 12 to 24 hours before pickling them. Excess lime absorbed by the cucumbers must be removed to make safe pickles. To remove excess lime, drain the lime-water solution, rinse, and then resoak the cucumbers in fresh water for 1 hour. Repeat the rinsing and soaking steps two more times.”  United States Department of Agriculture (USDA). Complete guide to home canning. Agriculture information bulletin No. 539. 2009. Page 1-28.
The USDA goes on to advise handling the resulting limed cucumber pieces carefully in order not to break them: “Handle carefully, as slices will be brittle.”  Ibid, Page 6-9
So Easy to Preserve (SETP) says,
“The calcium in lime does improve pickle firmness. If you choose to use lime, purchase food-grade pickling lime from your grocer’s shelves. Do not use agricultural or burnt lime. Food-grade lime may be used as a lime-water solution for soaking fresh cucumbers 12 to 20 hours before pickling them. However, (Ed: emphasis theirs) EXCESS LIME ABSORBED BY THE CUCUMBERS MUST BE REMOVED TO MAKE SAFE PICKLES. To remove excess lime, drain the lime-water solution, rinse and then re-soak the cucumbers in fresh water for 1 hour. Repeat the rinsing and soaking steps tow more times. Failure to remove lime adequately my increase the risk of botulism.”  Andress, Elizabeth L. and Judy A. Harrison. So Easy to Preserve. University of Georgia Cooperative Extension. Bulletin 989. Sixth Edition. 2014. Page 126.
The Ball / Bernardin Complete Book brooks no thought of using Pickling Lime.
“Some older recipes call for adding pickling lime, alum or grape leaves to pickles to make their crisper. Because these agents may have negative side effects, we have not used them in any of our recipes.” Kingry, Judi and Lauren Devine. Ball / Bernardin Complete Book of Home Preserving. Toronto: Robert Rose. 2015. Page 289
The Ball Blue Book (37th edition, fall 2014) and the Bernardin Guide (2013) don’t have any recipes calling for pickling lime, or any advice on its use.
Noted picking author Linda Ziedrich does include a few recipes calling for it, but with a proviso to only use it for recipes that allow for it and thus compensate for its possible lowering of acidity. She writes,
“The calcium in pickling lime is absorbed into the tissue of a vegetable or fruit, where it combines with pectin to form calcium pectate. This makes the pickles — usually cucumber, melon, or green tomato — firm and crisp. Southerners, especially, have long loved lime pickles. In the 1970s and 1980s, when natural foods were in, pickling lime was out, at least among recipe writers. But commercial food processors never stopped using lime, for much more than pickling. They add lime to canned tomatoes, potatoes, green beans, peas and many other foods. Provided it is food-grade (don’t buy it at a lumberyard), lime is a harmless food additive, but it can raise the pH of a pickle just as it can that of your soil.
So, after vegetables and fruits are soaked in a mixture of about 1 cup pickling lime to 1 [US] gallon water for 12 to 24 hours, they should be soaked three times in fresh water for an hour at a time and rinsed well after each soaking.
Even when you follow this rule, the pH of your pickle may be slightly increased, so use lime only in recipes that call for it.”  Ziedrich, Linda. The Joy of Pickling. Boston, Massachusetts: The Harvard Common Press. 2009. Page 17-18.
Pickling lime can produce pickles that are so crispy they are brittle and break.
You can still produce really crispy pickles that will be mouth-watering crowd pleasers without it: just use vegetables as absolutely fresh as you can (especially in the case of cucumber), chill them first before starting, and in the case of cucumbers, trim off the blossom ends where the softening enzymes live (trim off both ends if you are not sure — just a razor-thin shaving is all that’s needed.)
For a thorough discussion of crisping pickles via other means, see: Crisping Pickles on healthycanning.com.
Recipes calling for pickling lime
Here are some tested pickling recipes from reputable sources that use pickling lime (as of 2016)
USDA Complete Guide to Home Canning, 2015 edition: Bread and Butter (page 6-9), Quick Sweet (page 6 – 12), and Jalapeno Pepper Rings (Page 6 – 23).
Linda Ziedrich, The Joy of Pickling, 2009: Bread and Butter (page 90), Cantaloupe Pickles (page 260), Fig Pickles (page 261), Green Tomato Pickles (page 127).
So Easy to Preserve: Bread and Butter (page 131), Quick Sweet (page 135), Jalapeno Pepper Rings (page 148).
A commercial product, calcium chloride (sold as Pickle Crisp® by Ball / Bernardin) can help enhance the texture of vegetables for pickling (though it cannot restore lost crispness in wilting vegetables.) It is very easy and safe to use: it is usually added in small amounts to the jars as they are being filled.
The Ball Blue Book last had a recipe calling for pickling lime in 1979 (which was a reprint of the 1977 30th edition). Their “Crystal Pickles” on page 36 called for ⅓ oz (9 g) of it, with the following usage directions:
“Dissolve lime in water. If brine is cloudy, pour through a cheesecloth. Pour brine over tomato slices; let stand 24 hours stirring occasionally. Remove. Rinse well through several cold waters to remove all lime sediment; soak in cold water for 4 hours; change water each hour; drain.”
There have been no further recipes for pickling lime usage in the Ball Blue Book editions since 1979.
|↑1||United States Department of Agriculture (USDA). Complete guide to home canning. Agriculture information bulletin No. 539. 2009. Page 1-28.|
|↑2||Ibid, Page 6-9|
|↑3||Andress, Elizabeth L. and Judy A. Harrison. So Easy to Preserve. University of Georgia Cooperative Extension. Bulletin 989. Sixth Edition. 2014. Page 126.|
|↑4||Kingry, Judi and Lauren Devine. Ball / Bernardin Complete Book of Home Preserving. Toronto: Robert Rose. 2015. Page 289|
|↑5||Ziedrich, Linda. The Joy of Pickling. Boston, Massachusetts: The Harvard Common Press. 2009. Page 17-18.|