There are lots of factors that go into the need of chemical preservatives in soap. There are many that will be quick to say that since water is present in LS, it automatically needs one. The chemistry of soap plays major roles, and counters this argument. First off pH is the most known factor. The microbes we are concerned with cannot survive in the alkaline environments found in unaldultered, properly made soap. Concentration of soap in water is another. Glycerin content. For those of us that employ the glycerin method in LS making, we use quite a bit of glycerin in this method. According to the FDA (United States) when a product contains 10-50% in glycerin content (recipe dependent), it is considered self preserved. Even if you don't use glycerin method, you could easily add glycerin to be within that range.
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ADEQUACY OF PRESERVATION
Cosmetics need not be sterile, however, they must not be contaminated with microorganisms which may be pathogenic, and the density of non-pathogenic microorganisms should be low. In addition, cosmetics should remain in this condition when used by consumers. Some cosmetics, i.e., those containing more than about 10% ethanol, propylene glycol,
glycerol, etc., and cosmetics in self-pressurized containers, are self-preserving and are not likely to become contaminated with microorganisms."
http://www.fda.gov/ICECI/Inspections/InspectionGuides/ucm074952.htm
" Products in multiple-dose containers shall contain a preservative, except that a preservative need not be added to Yellow Fever Vaccine; Poliovirus Vaccine Live Oral; viral vaccines labeled for use with the jet injector; dried vaccines when the accompanying diluent contains a preservative; or to an Allergenic Product in 50 percent or more volume in volume (v/v)
glycerin."
http://www.accessdata.fda.gov/scripts/cdrh/cfdocs/cfcfr/CFRSearch.cfm?fr=610.15
I include citation on vaccines because quite honestly, if it's ok for it to be injected directly in our blood stream, it's ok for use as such in a wash off product.
Another large factor is that we cook our soap in all stages, including dilution. Or in the least, use boiled water. This essentially wipes the slate clean of microbes. GMP comes to mind as well, because it's stressed to always use clean, boiled water to begin with. But also celan utensils. Some of us also employ the use of alcohol in our soap for clarity or for making shower gels using Failor's alcohol method, or some modified method using alcohol.
I'd like to also add that some folks might use plant products in their soap, ie, aloe, and thus would require a chemical preservative. Not so, in my research based opinion. Referring back to all the reason's i mentioned, microbes are not an issue here. You'd only need to consider oxidative degredation that can come with using any plant based additives. A simple antioxidant, either natural or chemical, will do the trick here, and many antioxidants, as we know, do carry antimicrobial properties to some extent. There are also chelating agents that we all use reglarly in LS. Citric acid is your friend. Potassium citrate, should you not wish to tamper with the pH severely, is another.
So, with any combination, or all of these factors in place, there should be no worry about the need of a chemical preservative in LS. But then, we always revert back to the "cover your arse" logic. In that case, it's best to use a chemical preservative that is actually compatible with LS. Quite simply, you an't use any ol' product and expect it to be effective. LS is a special beast, and requires special treatment. A broad spectrum chemical preservative with pH restrictions that extend into the high alkaline range is needed. The following is what I've been able to find as acceptable for this, with their max pH limitations included:
https://docs.google.com/file/d/0B3e5_Ew1Rt6fdkJmckUtaldObG8/edit
Optiphen MIT Plus- up to 10
Suttocide- up to 12 (formaldehyde donor)
Aspen Bark Extract- up to 9 (found this via The Herberie and Ingredients to Die For)
You're not left with much to work with, are you?
I heard that Liquid Germall Plus is effective at higher pH ranges, however this contraindicates manufacturer suggestions, which suggests a pH limit of 8, according the the chart I linked just above. I'd stick to what the manufacturer suggests in this case.
When deciding to work with a preservative that has pH limitations, i recommend acquiring a reliable method of pH testing, to ensure accuracy and efficacy. Phenol-P drops are no good in this case, as they only indicate a broad range. Strips are ok, so long as they're lab quality, plastic type. You can even find some that are in half point increments. The next most certain suggestion is a good meter. But I understand money constraints in this case. So your fall back are pH strips.
I do hope this has all helped solve the matter at hand.
