I have to admit now that I have been a bit nervous about our new chalk lime plaster since last August, though I haven’t confessed it so far to the blog: we used imported pigs hair for the plaster in 2013, and more than a year after it was done I was speaking to another lime specialist who gave a deep intake of breath, frowned, and asked: has it fallen off yet? He seemed to relish recounting the story of another Suffolk house where the pig hair had disintegrated and the plaster coat had failed in the first year, on his version of the story because it had a foreign bug that had eaten it.
I’m now much more confident that our pig hair plaster is OK. There are no signs of failure, and from time to time over the autumn I pulled out a bit of hair near the surface and checked it for strength. I tried again this morning and the hair was as strong as ever.
I’ve now discovered more about the pig hair issue in a new briefing on lime which has arrived with the latest issue of the magazine of the Society for the Protection of Ancient Buildings, of which we are members. An article by Joe Orsi confirms that three years ago it was noticed that there was a pattern of render failure caused by degradation of animal hair. Samples were examined by a member of the Building Limes Forum, and it was found that some of the hair imported from outside the EU had been aggressively treated with acids to kill anthrax spores; the hair had been severely weakened because the keratin that binds it together was damaged.
More alarmingly, further tests with some locally sourced, untreated hair have found similar patterns of failure. Mr Orsi also says that there is evidence of hair failure in the past, but other hair has been unaffected and it is not clear why. (In the context, I assume that this is about hair that starts in a damaged state, rather than the well known problems arising when unused hair and lime mixes are allowed to stand for too long, which dissolves the hair). He goes on to discuss alternatives to hair such as artificial fibres, which we subsequently used for indoor plastering.
As I have mentioned in earlier blogs, when we first got involved with repairing a very old building 8 years ago there was great enthusiasm for complete authenticity, which meant we were encouraged to use ancient techniques wherever possible. But even at the time, revisionism was setting in, with a growing appreciation that there could be drawbacks to old methods and materials that in some circumstances make it better to mix ancient and modern. The discovery that animal hair can fail suddenly highlights this. I’m sure that if lime had not fallen out of use and then been revived after a long gap, there would have been craftsmen warning many years ago about the various ways hair mixes can fail.
The importance of lime remains absolutely clear, as does the advice against using cement in any building originally constructed using lime. But the more we learn about older methods, the more it seems we can find modern techniques to overcome some of the drawbacks.
I recommend the SPAB lime briefing for getting up to date on what the specialists are saying, including modern lime products such as Hempcrete and Limecrete.