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  1. #1
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    Default Chemical process of lime putty for fresco

    I am trying to get a clear picture of the chemical process the lime putty goes through as it ages. My understanding is this...that as the putty ages the water bonds become weak or perhaps the bonds break and water rises to the surface. But if that is the case, then wouldn't we get "fat" putty if we just added less water when we re-hydrated it? Also, how does aging increase the flexibility and elasticity of the putty? Thanks for any and all answers!

  2. #2
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    Default The chemistry of aging lime putty

    Quote Originally Posted by margaret View Post
    I am trying to get a clear picture of the chemical process the lime putty goes through as it ages. My understanding is this...that as the putty ages the water bonds become weak or perhaps the bonds break and water rises to the surface. But if that is the case, then wouldn't we get "fat" putty if we just added less water when we re-hydrated it? Also, how does aging increase the flexibility and elasticity of the putty? Thanks for any and all answers!
    margaret,

    This is indeed a very interesting question -- one we have wondered about over the years, too.

    We have made an inquiry to a person we know to be the most informed about the history of fresco materials, and are awaiting an answer. Of necessity, we think the answer is going to be an opinion, since we have never seen a published paper on this subject; however, we could surely be wrong about that, since we can't see everything!

    We can say that whatever the advantage of long aging of the slaked lime, it's likely tied to the chemical reactions it undergoes in storage in that below-the-frost-level pit. The Roman Italians like to specify that the slaking should take 25 years, and modern plasterers think 4 months is enough. Which is better, if either, and why, is your question.

    We also do not think that adding less or more water at the end of the aging process will have a good effect, because we are supposed to use the putty as it comes from the pit -- adding the necessary ingredients for the different coatings and little else. In our experience, the best aged slaked lime we have used will stick to a trowel even when the trowel is held upside down.

    We promise to re-visit this question when we hear from our colleagues.
    The AMIEN Staff

  3. #3
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    Default Update on source material

    Quote Originally Posted by markg View Post
    margaret,

    This is indeed a very interesting question -- one we have wondered about over the years, too.

    We have made an inquiry to a person we know to be the most informed about the history of fresco materials, and are awaiting an answer. Of necessity, we think the answer is going to be an opinion, since we have never seen a published paper on this subject; however, we could surely be wrong about that, since we can't see everything!

    We can say that whatever the advantage of long aging of the slaked lime, it's likely tied to the chemical reactions it undergoes in storage in that below-the-frost-level pit. The Roman Italians like to specify that the slaking should take 25 years, and modern plasterers think 4 months is enough. Which is better, if either, and why, is your question.

    We also do not think that adding less or more water at the end of the aging process will have a good effect, because we are supposed to use the putty as it comes from the pit -- adding the necessary ingredients for the different coatings and little else. In our experience, the best aged slaked lime we have used will stick to a trowel even when the trowel is held upside down.

    We promise to re-visit this question when we hear from our colleagues.
    margaret,

    We may have found a published source that gives the information you seek, up to a point, and we are awaiting confirmation that we've found the right article.

    The problem for us artists is that this sort of information is sometimes only found in small, low-circulation specialty journals or other publications that are normally only accessible to those who are "members of the club," so to speak. But we have our ways ...
    The AMIEN Staff

  4. #4
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    Default chemical process of lime putty

    I agree it is difficult to find resources, a lot of digging needs to happen. C. Rodriguez-Navarro is a name that keeps popping up from the Getty Conservation Institute. I have been able to access a few of his articles and from what I understand, the molecules undergo a chemical change in size and shape making the putty more compact and thick. But is water released in the process??

  5. #5
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    Default The resource, found

    Quote Originally Posted by margaret View Post
    I agree it is difficult to find resources, a lot of digging needs to happen. C. Rodriguez-Navarro is a name that keeps popping up from the Getty Conservation Institute. I have been able to access a few of his articles and from what I understand, the molecules undergo a chemical change in size and shape making the putty more compact and thick. But is water released in the process??
    margaret,

    We found the most definitive work on this subject, to date. The problem for artists is that these publications are not often easily accessible to us, even though they may contain valuable information. You either have to be a member of the organization that publishes them, or be a member of "the club" (so to speak) for easier access. As it turns out, the ICA has a subscription to "Studies in Conservation," so that solved the problem for us.

    “Lime Putties and Mortars: Insights into Fundamental Properties,” By Eric F. Hansen, Carlos Rodríguez-Navarro and Koenraad Van Balen. The Journal of the International Institute for Conservation of Historic and Artistic Works (IIC): "Studies in Conservation," vol. 53, no. 1, (2008), pp 9 - 22.

    From the Abstract: “The calcium hydroxide crystals in slaked lime putty may be further modified upon storage in water for extended time periods, also affecting the working properties and density.”

    The paper goes on to recommend more studies related “ … to application, … drying rates, shrinkage, cracking, carbonation rates, porosity and strength development, and durability.”

    “Differences in lime putty properties can be due to a wide variety of factors other than the method of hydration, such as raw materials (e.g., type of limestone calcined) and processing (e.g., over-burning or under-burning, extent of hydration achieved during slaking, hydration and carbonation of quicklime during storage), among others. … well-slaked lime putty exhibits colloidal behavior, and its plasticity is highly dependent upon the abundance of colloidal and hexagonal plate-like particles. These plate-like particles have a great capacity to absorb water and affect not only the plasticity but also the working properties in general, including viscosity and water retention.” In other words, aged slaked lime putty is better.

    A good quote: “ … practice is not always matching theory,” from another paper, Teutonico, J.M., et alia, “The Smeaton Project: Factors affecting the properties of lime-based mortars,” "APT Bulletin" vol 25 (1994) pp 32 – 49.

    In the end, the authors only conclude that “aging lime putty for extensive periods (years) … may contribute to enhancing its quality …”

    We recommend you find the article yourself -- a good reference library might have it -- and discover useful information for yourself. As far as we are concerned, this article seems to say that aging slaked lime putty for a long period is a good idea.
    Last edited by markg; 09-27-2008 at 04:14 AM.
    The AMIEN Staff

  6. #6
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    Default

    Excellent! Thank you so much for the information regarding the resource. I look forward to finding and reading it...
    All my best,
    Margaret

  7. #7
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    Default The resource, found

    Quote Originally Posted by margaret View Post
    Excellent! Thank you so much for the information regarding the resource. I look forward to finding and reading it...
    All my best,
    Margaret
    margaret,

    We admit it was fun to track down that article.
    The AMIEN Staff

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