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Glycerine is a common ingredient in bubble recipes and one of the most misunderstood. It is very useful for helping mix polymers (since most of the useful ones are insoluble in glycerine), and, in some cases, can extend bubble life. However, it has earned a place in the popular mind as a magic ingredient in bubble juice. And, while it is sometimes useful, it is an ingredient that often has no (or minimal) influence on bubble juice.
Somehow, it has taken a hold on the imagination of so many people that even serious bubbleheads include glycerine in their recipes even though their recipes seem not to suffer when it is removed.
It is my opinion (with no notable dissent from other people that have performed careful tests) that glycerine will not improve the longevity of giant outdoor bubbles in most cases. It is very useful for mixing ingredients and may also improve the shelf-life of some mixes since glycerine can preserve the viscosity of some polymers (EDITOR: find the reference from Dow about this).
Various (and quite different) claims are made about the actual role that glycerine plays. The amount of glycerine used varies widely in recipes found on the web. Some recipes use as little as 1/4 part glycerine (where 1 part is defined by the amount of detergent used) and some use as much as twice the amount of glycerine as dish detergent. One chemist has suggested that the amount of glycerine needed is relative to the pH of the detergent--with high pH detergents requiring relatively large amounts of glycerine. This claim has not been subtantiated.
One experiment indicates that small amounts of glycerine have little effect in at least some formulations and that high impact may require much larger amounts of glycerine than found in many common recipes. For large outdoor bubbles, it appears that under most conditions with the most commonly used dish detergents that glycerine has no measurable effect even in large quantities. (July 2012: There are currently no reliable reports or tests that contradict the belief that glycerine does not extend bubble life of outdoor big bubbles).
The information that I have found about the pH of glycerine is contradictory--if anyone knows the reason for the discrepancies, please update this section. The pH of Starwest Botanicals vegetable glycerine was measured with a Milwaukee ph600 pH tester to be 5.6 (the unit is accurate to plus/minus 0.1). Keith Johnson also found glycerine to be acidic. However, several web sites indicate that vegetable glycerine is pH neutral (7.0) (in which case the acidity of the purchased glycerine could be due to the water used in its preparation -- since distilled water tends to be slightly acidic due to dissolved carbon dioxide from the air) while others indicate that it is basic (10.0). [NOTE ADDED Sept. 2010: I just remeasured the bottle of Starwest Botanicals with a freshly calibrated pH meter. The pH was 7.2. It isn't clear whether the earlier 5.6 reading was due to a malfunctioning meter or if the pH has changed over time. Tonight, two other brands of gycerine were also tested with results ranging from 6.3 to 7.3].
Some experiments by Edward Spiegel indicate that KY Jelly-type lubricants seem to react with the glycerine and detergent to form solutions that are significantly more bubble-friendly than solutions that contain only glycerine or only personal lubricants. However, this synergistic effect seems only to be apparent in very concentrated solutions with little water. In these tests, many more bubbles per dip were created when the recipe included both glycerine and a ky-jelly type lubricant than in those that had only one of those ingredients. Those tests were done with Ultra Dawn (2010 formulation) detergent and tap water. It may be that the glycerine enables the creation of long molecular chains. At standard dilutions, the glycerine does not seem to affect bubble-friendliness except that it does significantly extend bubble life and stabilize colors when there is enough glycerine. Ongoing experiments will be documented in Edward Spiegel's Bubble Blog
Some detergents, such as Dawn Direct Foam, seem to have increased bubble potential for small-wand-bubbles when glycerine and little are no water is added.
Brian Lawrence says that experiments done with the old Dawn formulation (which was changed in the 1990s) indicated that when relative humidity was high (over 92%) that glycerine could nearly double the lifetime of a bubble but that this effect dropped off rapidly below 92% humidity. In those tests, at 100% relative humidity, there was a linear correlation between bubble longevity and the amount of glycerine up to about 7 grams of glycerine per liter. More glycerine did not seemt to hurt the bubbles which were tested with amounts up to 21 grams/liter, but they did not benefit them either. It remains to be seen whether this effect is related to the detergent with which it is used. This is an area that needs to be explored more fully. Note that these tests were done indoors and don't appear to contradict, the conclusion that glycerine has little impact on giant outdoor bubbles.
It remains to be ascertained which factors influence how much effect glycerine will have on a mixture and which aspects of bubble-making are impacted. Bubble longevity is definitely impacted when there is enough glycerine even in humidity of 40% to 50% at least in some recipes. It is possible that the effectiveness may depend on the particular detergent being used and the other amendments.
Some bubblers notice little effect of glycerine in their solutions. This lack of impact can be for a number of reasons -- in many circumstances bubbles will be popped or broken before their natural lifespan has been exhausted. In such cases, the glycerine impact would not be noticed. Out in the environment a difference in lifespan of 30 or 40 seconds might not be noticed and a very large amount of glycerine is needed to extend the lifespan longer than that (except in the highest humidity). Few recipes call for the amount of glycerine that would be needed for such an extension as documented in these tests..
1 fl ounce of glycerine weighs about 37.3 grams. 1 ml of glycerine weighs about 1.26 grams.
Tom Noddy posted this interesting message to SBF, the Soap Bubble Fanciers Yahoo Group (RIP) on the topic of glycerine.
- The idea that glycerin is a magic ingredient for bubbles pre-dates the era of Dawn as a central ingredient.
- To my knowledge, the superior qualities of Dawn over the other available
detergents was popularized when the Exploratorium's Ned Kahn and I separately
tested many many products in an effort to replicate the big bubbles of Eiffel
Plasterer for use in the new bubble exhibits that were being designed in 1983
for the first ever Bubble Festival at that San Francisco science museum. (All of
those Kid-In-A-Bubble exhibits and the large bubble wall exhibits at science
centers and children museums all date from those efforts we made then to add to
the hands-on aspect of the upcoming Bubble Festival that I talked them into at
that time).
- To my knowledge, no one was particularly interested in turning Dawn into a
useful mix for small bubbles back then ... Wonder Bubbles/Mr. Bubbles was
already good and very cheap and available. It was this pursuit of big bubbles
that led to the recognition that Dawn is the good stuff.
- CV Boys used glycerin and it is often referred to in the recipes offered by
scientists in their experiments with soap bubbles. I agree with you, Edward,
that its usefulness in the modern recipes is limited and I certainly agree that
it is the one thing that most people "know" about good bubble solutions. I tried
it, even in the early Dawn days, and didn't bother with it for long. It could be
that it was a much more useful product in overcoming disadvantages of soap,
rather than detergent.
- Tom Noddy
www.bubblemagic.com
See Controversies and Open Questions Also Small Wand test results for concentrated detergent/glycerine combinations Also Variations of Glycerine pH (blog entry)