#AskAConservator 2022

In general, if a building was not built using a certain material, then it shouldn’t have that material introduced into it unless there is a documented need and testing shows the new material is compatible with the old building’s systems and materials. There are times when shifts in buildings cause engineering considerations that necessitate the switch to a stronger mortar, but those should be evaluated on a case-by-case basis and not taken as a general rule.  Wood expands and contracts at a much greater rate than Portland cement, so you tend to get cracking and separation at the place the wood and cement meet. These voids then allow water to penetrate deeper into the material, which in the case of wood can cause root or contribute to termite infestation. Additionally, Portland cement also has a higher capillarity and lower vapor transmission rate (sometimes called “breathability”) than lime, which further can exacerbate moisture issues. This is usually why you need special materials like mastics (traditionally) or caulks (modern) to help separate places where wood and masonry/mortar meet. Traditional chinking uses lime, clay, straw, and sometimes oils like linseed or hemp oil to help make a more flexible and water-resistant mix.  We have a good article detailing this process on our blog here:

https://www.limeworks.us/log-chinking-ye-alde-calk-mend-the-gap-between-brick-stone-and-frames/

Good question!  For the most basic evaluation, which is really just a few basic physical tests and a judgment call based on the evaluator’s experience, you need a solid piece of mortar or several smaller solid pieces amounting to about the size of a human thumb.  For laboratory testing, the amount you need really depends on the test being performed.  Basic acid digestion tests require about 30g or so of mortar, which is generally the amount that can fit in a human palm.  The more tests you are doing, such as the instrumental tests that are performed in ASTM C1324, the more material you need.  In-depth Instrumental Analysis requires at least 100g of material (about two palmfuls).  But more is better, because sometimes a test needs to be done more than once to account for mistakes or unclear results. It’s absolutely vital to send solid samples, as these tests cannot be performed accurately on powder.

When you can’t get a good mortar sample and even when you can, testing the masonry units is definitely useful.  The properties of the masonry units (brick, stone, etc.) are often just as important or more so than the properties of the mortar, as the mortar is meant to be sympathetic to the properties of the masonry. So, knowing properties like compressive strength, density, and water absorption rate of masonry can give you a really good picture of what kind of mortars are plausibly compatible.  Remember when testing masonry units, more is better since masonry, especially brick, can vary a lot from unit to unit.  It’s good to test a minimum of three bricks selected from random areas around where you are repairing, but if you can do more it’s even better.

I suspect your emphasis on the quality of the brick may be a red herring.  Modern brick is generally much more durable than any historic brick on average.  The problem is that our mortars are extremely dense and hard compared to historic mortars. A modern brick will typically have between 500-1500 psi of strength depending on the class of brick it is. But modern mortars can get much, much stronger than that.  ASTM C270, which governs the strength of masonry mortars, only specifies minimum strengths.  Consider that a Type N mortar (your average every day laying and pointing mix) has a minimum strength at 28 days of 750 psi. So you might think, “My brick is 1100 psi, 750 sounds fine!”  But, modern Type Ns in reality reach around 1000-1500 psi of strength at 28 days and continue to gain strength beyond that for several years (as all mortars do).  So, even at 28 days, the mortar is within spitting distance of the strength of the brick  By 2-5 years, it could very well be stronger and denser than the bricks, which can lead to all kinds of problems, especially in salt-laden locations, wet environments, or places with frequent freeze-thaw cycling.  And that’s just Type N, the “middle of the road” kind of Portland cement mortar.  Stronger mortars that are also routinely used like a Type S can be even more problematic as they get even stronger and denser and are used in more extreme environments.  So even a modern brick can have problems with a Portland cement mortar!  Gentler lime-only mixes are not typically used in modern construction, so it could very well make you think modern bricks are inferior when in reality the problem lies in the mortar!

Lime-only mortars are largely off the radar of most American organizations, including ASTM, as we’re still struggling with lime vs. cement in the broad sense.  I cannot disclose specific conversations or work items that ASTM has active, but generally speaking, I’m not aware of any major push towards additional standards for lime mortars.  Standards are needed and will likely be our next frontier within the coming few years as their use becomes more popular in the US and Canada and blindspots in the current standards become more apparent.

To the broader question of NHL 3.5 vs. NHL 2, it really depends on the context.  There are places where NHL 3.5 is indeed too strong, but I’ve found this to be the exception rather than the norm in the US.  In general, though, one problem with NHL broadly is that there are so many manufacturers and NHLs of different chemistry or mineralogy can often have “runaway strength” where they get much stronger than they are advertised as.  This is one reason why we at LimeWorks prefer Saint-Astier NHL, as it is very well studied and very consistent in its strength.  That said, I think the argument of lime vs. lime (usually framed as hydraulic vs aerial lime) is a bit overblown and overdramatized in some circles and is being pushed mostly by people with a vested interest in one manufacturer and/or variety of lime or another.  More knowledge is always good, so I’m 100% on board to do more research on the nuances between lime types and how those matter to the average user, but my own research and experience have led me to believe that in most cases consistent and well-studied varieties of NHL 3.5 are an acceptable substitute for historic lime mortars on most average historic masonry.  However, no matter what anyone says, one should always do one’s diligence to test mortar and masonry on historic buildings first and only choose well-researched and documented lime products that have unbiased third-party testing that shows they are plausibly compatible. As lime has become more popular, there are a lot of unproven products entering the market promising miracles they are not necessarily capable of delivering.

The other aspect of this question is realistic implementation of aerial limes in major construction.  In the US, it’s difficult to get projects, even those on historic buildings, to allocate the proper time for an NHL to cure.  If we were to move towards aerial limes only as some proponents would like, we’d likely lose a large share of the market as aerial limes require much longer after care times in a market where 72 hours is already seen as excessively long. The solution to this is usually to add pozzolans to aerial limes to speed up the set by making the lime hydraulic, but at that point, you’re using a product that is even less predictable and understood compared to NHLs, so it nullifies the point. Indeed, studies of historic limes show that aerial limes were very frequently modified with pozzolans making them hydraulic, so the use of “pure” aerial limes is probably not as common as we think.  All that said, lime of all types needs a bigger market share not just for historic masonry but also for modern construction in order to drive down the overall carbon footprint of the building industry, but NHL in particular provides a good compromise between ease of use, consistency of results, and sympathy to most building materials.

Dealing with crumbling bricks is always difficult.  There are products on the market like our Waterglass that are silicate consolidants which can help densify the surface of dusting mortar and brick, but they are only going to work to an extent.  When repairing lost faces or other details on a brick, you can also use a restoration mortar like our Lithomex to rebuild portions of the brick.  However, when we’re talking about a brick that is compromised significantly, there usually aren’t good cost effective options other than to do a full unit replacement.  When it’s one brick here or there that has lost less than 50% of its volume, Lithomex and Waterglass are good options, but if you have a large significant area of brick loss, it is possible that the wall can be compromised.  You’d need a structural engineer to assess in such cases and would likely need to provide structural support to the wall while the decayed section is excised and relaid with new or salvaged brick.  I’m reluctant to give broad strokes here on the whens, wheres, and levels of intervention beyond that because each situation and building is different.  I’d recommend reaching out to us or another qualified conservator to help you assess these kinds of situations on a case-by-case basis to better understand what may be needed or not needed.

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In an ideal world we’d test all mortars and masonry before doing work.  But in most cases, no, a basic evaluation to get a sense of the mortar is probably realistically sufficient as not everyone can afford more in-depth analysis.  However, there is always a certain degree of risk one must accept if not doing more objective scientific studies as basic tests like using vinegar to see if it fizzes or a simple hardness test are not going to actually tell you anything absolute and are highly dependent on the skill of the evaluator to interpret the results.  On the other hand, you can also sink thousands of dollars into very precise scientific tests only to get the exact same product recommendation you’d have gotten from a basic evaluation.  It will be up to each person to evaluate how much risk they are willing to accept and how much of their budget they want to devote to understanding vs. fixing.  In the end, any testing is better than just going down to your local hardware store, buying a standard bagged mortar, and applying it without ever thinking through the materials already on the building.

Without having engaged with the source material you’re referencing, I cannot really provide a good response. In general, though, I certainly welcome having a greater selection and knowledge of aerial lime mixes available to the US.  Aerial limes, when applied and cared for, are great mixes–not appropriate everywhere, but excellent when deployed tactically in appropriate contexts (but this is true of any material). There is such a thing as too much porosity and too much vapor permeability.  One issue aerial limes can have, especially when applied improperly or when hydrated lime is used instead of authentic quicklime putty, is that they let too much moisture too far into the material into places where the masonry has not been exposed to weathering, which can cause new damage.  Likewise, hydraulic limes can also cause damage if used improperly, especially when lower quality NHLs or PHLs are used as their strengths are much less predictable than more researched and controlled brands. However, the SPAB does not discourage the use of hydraulic lime–quite the opposite. Their website’s technical advice section on lime acknowledges that hydraulic lime was used historically (and even preferred in many cases), and that both hydraulic and aerial limes have a place in historic preservation.  It’s been my experience that anyone who pitches the absolute superiority of one kind of lime over another in all situations as a general rule usually has a market interest they are not disclosing. The best practice is exactly what SPAB’s website and our own US Secretary of the Interior’s Standards recommend–research what was plausibly available at the time of construction, test the materials present, and avoid things that weren’t unless circumstances dictate a deviation from this and testing shows your replacement material won’t harm or disrupt the building’s systems.

Limewash is generally a very safe product for historic masonry, but whether or not its “good” depends on the situation. There are a lot of myths out there about limewash as this miracle coating will solve your moisture issues, prevent all mold growth, and act as a shield to your masonry.  These are a bit overblown and largely mythological.  Limewash is a great evaporative surface to help move salts and moisture away from the surface of your masonry to delay damage or just to brighten up your space, but it isn’t going to turn a wet basement into a dry one by itself or stop all bio growth forever.  Likewise, there are limitations to limewashes.  They aren’t great on already-painted/sealed surfaces and don’t adhere well to non-masonry materials like wood without modification.   Without knowing more about your specific context, I can’t say for certain if limewash is a plausible solution for you, but I’d be happy to talk to you more about it.

For homeowners or professionals looking to learn the basics of working with lime mortar, we offer several classes on the topic.  You can also send us a sample of your mortar and we can advise you based on that.  There are a few general principles you should always follow though: 1) Don’t ever use anything in the building that was not there originally unless there is a reason to and testing shows it won’t do harm; 2) Use materials that are sympathetic (sometimes called “like-for-like”) to the original material and the surrounding masonry; 3) Mortars should be less dense and have greater vapor permeability than the surrounding mortar.

When looking for a craftsperson, you should look for someone who has documented a successful history of using traditional lime mortars on buildings like your own.  Generally, you want someone who has several years of experience, not a newcomer to the material.  Masonry experience with other materials like Portland cement can translate over to lime mortars pretty quickly for a mason willing to learn, but just having experience with Portland mixes is not a substitute for experience with lime-only mixes as there are differences in them, especially in the aftercare.  Check their references and insurance to make sure they are qualified to do the work you are hiring them to do and covered in case it goes wrong.  Finally, if you want a little extra help, we host a project finding service at Craftworker.net where you can post your project for free and have masons in your area that use the service get a notification to contact you if they are interested.  Craftspersons using this service are fully independent of us and we do not guarantee, insure, or employ them, but it can be a head start in finding someone if you need a bit of a boost.

Lime mortar can be used in cold weather, it just requires more work on your part.  There are a few tricks you can use to help make it work.  First, don’t use as much water when pre-wetting the wall or pre-mixing the mortar.  Make the mortar as dry as possible while still being workable.  Second, try using hot water to mix the mortar as this accelerates the set slightly and can also ward off cold temperatures for a bit longer.  Third, at the end of your work day, you can cover the work area with a thermal blanket or 6 mil plastic tarp held a few inches away from the wall.  Under the tarp, string 40-60 watt lightbulbs (not LEDs, real incandescent bulbs) and point them towards the work.  Keep these lights on as long as the temperature is below 40*F.  This will give you just enough warmth to resist temperature down to about 25*F degrees.  Remember when temperatures drop below 45*F, you should add 24 hours of curing time for every degree below 45*F

Good question!  “Softness” is usually a way of talking about density, porosity, and water vapor transmission (aka how “breathable” the masonry is).  In Portland cement mortars, the softer the mortar is, the less dense and porous it tends to be.  However, this way of thinking doesn’t as neatly apply to limes and clays.  In most average residential construction, there usually isn’t such a thing as a mortar that is too soft, at least when using typical building materials.  But there certainly can be such a thing as too porous or too vapor transmissive (“breathable”).  While we usually want breathable materials in traditional historic mass masonry walls, if a mortar is TOO breathable, it can allow water deeper into the wall than normal.  Why this is a problem is that salts and other minerals, and ice in cold climates, now has access to areas of the wall that have not weathered or maybe aren’t even designed to weather. This can lead to water reaching wooden members, condensation in the interior, or even accelerated weathering in areas where you didn’t previously have water problems.  Where you’ll often find this problem occurring “in the wild” is when people attempt to use hydrated lime-based mortars or lime putties that are substantially less dense than the historic mortars.  It’s important to try and replace mortars with materials that are similar to what was there originally as the wall system will have aged into this kind of system.  There are of course exceptions where you need to replace a mortar with something less or more dense than the original, but as a general principle, “like-to-like” is what you should aim for.