#AskAConservator Day 2021

We don’t recommend attempting to work with lime plaster or mortar when temperatures are going to fall below 45 degrees anytime in 72 hours after work is complete.  Proper pre-project planning can help ensure you are doing work doing the right seasons, but sometimes an emergency repair requires working in the colder months and so proper care and custody of the work is a must when working with lime or other traditional materials. 

If you have to work in sub-45 degree temperature, you need to provide an external heat source for the duration of that 72 hours.  This heat source can be as little as an incandescent light bulb (but not LEDs!), but you need to take care to insulate the area with tarps or heat-retaining blankets and direct the heat towards the work.  You also want to control the water amount in your mortar by pre-wetting only the absolute minimum and mixing the mortar as dry as possible while still being workable. 

Remember also that just because the air temperature is above 45 degrees, the masonry itself might be much colder, especially if you’ve had a stretch of cold weather already.  We recommend adding one extra day of curing time for every day below 45 degrees.  So if you were working in 44 degrees, 72 hours becomes 96 hours.  If you were working in 35 degree temperatures, suddenly you need almost two weeks of curing time providing an external heat source.  

There is no real way to stabilize deteriorated mortar.  If your mortar is crumbling and dusty, it’s time to repoint.  The specific kind of mortar you will need can vary depending on the stone, but in general, you’ll probably want to use an NHL 5 for a foundation as it is more resistant to water and salts.  However, if your limestone is particularly soft, you may have to compromise with a lower strength NHL  Wait at least 10 days, and ideally 28 days, before doing your parging with a similar strength mortar to the one you pointed with. 

The limestone falling away in sheets is called delamination and is quite frequently a sign that something is going wrong with the mortar or foundation system.  If the wall has ever been covered with a cement-based parging or an acrylic-latex paint, or if the mortar was replaced with hard and dense cement mortar, it is likely the damage to the stone is being caused by trapped moisture and salts blowing apart the bedding layers of the stone. 

Depending on how severe the damage is, there are several different approaches here which can include consolidation, surface patching, or even full unit replacement with new stones in severe cases.  You may need a professional to come out to the site to review the potential causes of the stone failure and help you plan an intervention.

“Working on a 200 year old building that has had its foundation encased in concrete to save it from collapse, but restoring everything around it with lime mortar. Not ideal situation, for sure.”

While we’d ideally like everything to be sympathetic, the reality is that some old buildings have many years of inappropriate or well-meaning repairs that simply cannot be removed due to safety, budget, access, etc.  Lime mortar works well in this situation because it tends to be less dense than most other kinds of stone or mortar, so it will draw water to move away from the other materials and hopefully also away from the sensitive historic masonry. 

It certainly isn’t a guarantee, and it is important you be honest with your clients so that they understand that if they don’t have or won’t provide the budget or permission to carry out removal of inappropriate repairs, they may experience continuing issues like further water damage to masonry, water infiltration, or quicker than normal weathering of the replacement lime materials near the inappropriate repairs. 

It can be tempting to think of materials like lime as miracle solutions that will solve all our modern problems, but really lime is a material tool like any other and has its limitations and appropriate and inappropriate uses.  Likewise, sometimes other values or circumstances of the site require a material to be used that may not be ideal in other situations. 

In the case of your 200-year-old building, the engineers obviously thought that the need to prevent collapse outweighed the risk of decay from inappropriate repair materials.  Perhaps having a different engineer look at it may find a different solution that wasn’t available or wasn’t seen at the time the first repair occured may find a way forward that balances preservation of the materials with structural stability of the building.

“I do restoration work in the Shenandoah Valley of Virginia. Our work is mostly carpentry-related but sometimes involves masonry repointing. We are currently repointing a 230-year-old limestone chimney.

After backfilling deep pockets with a weaker lime mortar mix we are doing the finish pointing with a mix of 2 masonry sand (orange) to 1 NHL 3.5 and a raised joint. I have done this many times in the past and the mix always seems sandier than what I am trying to match. It can be hard to blend in with the old mortar which seems smooth and creamy. The color looks good but the texture seems off.

The early German settlers in our came down the valley from Pennsylvania and I believe the early work here is probably similar to the Lancaster area.”

Early settlers often had to make do with what they had in the area, and so they would gather sand from local rivers or sandpits, and slake lime right on their property from local limestones.  This means there can be huge variation in vernacular buildings even from one side of a town to another depending on what local materials and local skills were available. Of course, this can make replication really tough because often we can no longer get sand this way, or even if we could the river has changed since Colonial times and may not be appropriate anymore due to modern runoff or changes in sediment.  This means you may need to use pigments or source special architectural aggregates from elsewhere in the country that better replicate the look.  But you do need to be careful, because whenever you introduce material that wasn’t there to begin with you may get material incompatibility problems. 

If using pigments, always use alkali and UV-stable pigments that won’t fade due to the sun or high pH of the lime.  Do small mockups off the wall first (like curing in a small disposable cup) to test each color blend and then tweak the mix by adjusting pigment or colored aggrate amounts through, frankly, a process of “guess and check.”  Make sure to plan enough time and budget enough cost to allow this process to reach completion, it isn’t something you can usually do off the cuff on the day of the job.  When in doubt, send a sample to us and we can help with a Custom Simulation.

There are several different kinds of decorative mortar joints you may be talking about.  You may be talking about a Ribbon Joint, a Grapevine Joint, a Raised Ridge Joint, or perhaps even some other rarer forms.  Check out our helpful visual dictionary of mortar joints and recipes on how to make them: https://www.limeworks.us/resources/recipes/

No, lime cures via a process known as carbonation.  During this process, the calcium hydroxide in lime absorbs carbon dioxide from the air and becomes a totally different, less reactive material known as calcium carbonate (literally limestone).  In hydraulic limes, water also triggers a set via the formation of calcium-silicate-hydrates.  Once the process of setting is complete, the lime can no longer be hydrated to form a reactive mortar or plaster. 

Without knowing more about your application, it is hard to say for sure.  But some common problems that result in plaster cracking are:

• Not adequately pre-wetting when applying directly to masonry.
• Mixing the plaster too wet or allowing it to get too wet after application (either through rain or over dampening).
• Adding too much or too little aggregate or the wrong kind of aggregate when mixing your own plaster from lime and sand.
• Not being careful to keep the work damp and cool for 72 hours after application.
• Applying in temperatures above 85* or below 45*F without protecting the work.
• Not ensuring your substrate is firm and non-flexing

If you are getting breaks on your plaster on a cob oven, there is a strong possibility you are lighting the oven when it is not fully dry or overwetting when applying the plaster.  Clay expands and contracts in the presence of moisture and if you put a fire in it, the water turning to vapor (or even steam depending on temperatures) can also crack the plaster surface as it forces its way out. 

Furthermore, because cob is very soft, you need to be careful what kind of plaster you use–even certain NHLs can be too dense and strong for cob.  Stick with Saint-Astier NHL 2 and no stronger, and you can mix fibers or hairs into the plaster to give it more tensile strength.  Wait a minimum of ten days before lime washing and do not light your first fire until the entire thing is well and dried out. 

There is no real way to stabilize deteriorated mortar.  If your mortar is crumbling and dusty, it’s time to repoint.  The specific kind of mortar you will need can vary depending on the stone, but in general, you’ll probably want to use a Saint-Astier NHL 5 for a foundation as it is more resistant to water and salts.  However, if your limestone is particularly soft, you may have to compromise with a lower strength NHL  Wait at least 10 days, and ideally 28 days, before doing your parging with a similar strength mortar to the one you pointed with. 

The limestone falling away in sheets is called delamination and is quite frequently a sign that something is going wrong with the mortar or foundation system.  If the wall has ever been covered with a cement-based parging or an acrylic-latex paint, or if the mortar was replaced with hard and dense cement mortar, it is likely the damage to the stone is being caused by trapped moisture and salts blowing apart the bedding layers of the stone. 

Depending on how severe the damage is, there are several different approaches here which can include consolidation, surface patching, or even full unit replacement with new stones in severe cases.  You may need a professional to come out to the site to review the potential causes of the stone failure and help you plan an intervention.

If you’ve lost more than 50% of the body of the brick, you should consider replacing those brick.  It’s great that you saved the brick from the chimney as those are likely to be very similar if they were made at the same time as the rest of the house. 

For brick with less than 50% loss, you can rebuild the face of the brick with a restoration patching mortar like Lithomex.  For brick that are dusty but not falling apart, you can use a consolidant product like Waterglass to help firm them up. For any mortars or parging material, make sure you are using a material that is softer and less dense than the brick.

Given how old your bricks are, I’d recommend having a minimum of three good condition bricks extracted and tested for compressive strength and water absorption to get a good sense of what kind of mortar strength you can plausibly use safely.  After that, there are several different kinds of lime-based products that we can prescribe to help control the vapor permeability of the mortar joints or parging to help preserve the brick.

Keep in mind though, that none of these interventions will solve the moisture problem.  You should take a look at your foundation grading, water table height, roof condition, and any downspouts to see where the water is coming from and take steps such as grading the foundation, repairing gutters, or installing a French drain and waterproofing to help direct water away from the very soft historic foundation material.  

The darkness is most likely being caused by moisture movement into the brick.  To match the brick effectively so that they get dark in a way consistent with the rest of the brick around them, you’d need to source bricks of similar vintage and density.  Check architectural salvage companies that specialize in bricks and you can often find reclaimed brick from demolished historic sites that will be compatible with your historic brick.

If you have any doubts about your foundation moving, you should consult with a structural engineer. They can do tests and monitoring to determine if the foundation is moving, and if so, whether or not it is a concern.

“I have been working with lithomex for a little bit. (Sandstone) we have tried brushing, some tooling, pressing stone pieces onto the lithomex surface but have not quite achieved making it look like actual sandstone.”

If you’re planning on working with Lithomex regularly, consider taking one of our Lithomex classes, or even getting certified as a Lithomex Certified Installer if you aren’t already.  One of the main tricks when replicating stone in Lithomex is paying attention to the layering and direction of the various colors and grains.  You may need to use multiple Lithomex colors blended or stacked while wet to get a convincing effect. 

For finishing the surface, there are a bunch of tricks, but one I like to use is just beating the surface up with a churn brush a little bit to make it look dimpled and pocked like a weathered stone.  I also use dental tools and even things like forks or spoons to carve out little bits of the surface and make it look like the natural formations of a piece of stone. 

It takes a lot of practice to make a convincing stone in Lithomex or any company’s patching material, so you’ll want to do small mock ups in your spare time and use reference photos or real physical pieces of stone to really hone your skills at a convincing replication stone.  Remember, sometimes you can have the right materials and right tools, but still make a repair look poor by not taking the time to master the art of surface replication!

“When we repointed- we were thinking of having a stone caravel with the date but thinking now maybe just making out of plaster. Any help and resources would be much appreciated.”

Any material you put in here is unlikely to have enough surface area compared to the whole mass of the wall to cause significant problems so long as you are using a good mortar that is less dense and more vapor permeable than the surrounding masonry and not putting a stone contaminated with salts or other harmful pollutants into the area. 

Ideally though, you’d want a stone that has similar or sympathetic properties to the rest of the masonry in the area as like-to-like materials have the best chance of not causing damage or being damaged.  “Like-to-like” means a similar compressive strength, density/absorbency, and mineralogy in this case.  A material too dense could cause the surrounding mortar and masonry to wear quicker, while a stone significantly less dense or more absorbent could wear quicker than the surrounding masonry. 

You may want to consider Lithomex, as it can be carved or cast and has properties similar to a sturdy limestone once cured.  If you are using plaster, remember that gypsum plasters are not for outdoor use generally as they are water soluble.

If you’ve lost more than 50% of the body of the brick, you should consider replacing those brick.  It’s great that you saved the brick from the chimney as those are likely to be very similar if they were made at the same time as the rest of the house. 

For brick with less than 50% loss, you can rebuild the face of the brick with a restoration patching mortar like Lithomex.  For brick that are dusty but not falling apart, you can use a consolidant product like Waterglass to help firm them up. For any mortars or parging material, make sure you are using a material that is softer and less dense than the brick.

Given how old your bricks are, I’d recommend having a minimum of three good condition bricks extracted and tested for compressive strength and water absorption to get a good sense of what kind of mortar strength you can plausibly use safely.  After that, there are several different kinds of lime-based products that we can prescribe to help control the vapor permeability of the mortar joints or parging to help preserve the brick.

Keep in mind though, that none of these interventions will solve the moisture problem.  You should take a look at your foundation grading, water table height, roof condition, and any downspouts to see where the water is coming from and take steps such as grading the foundation, repairing gutters, or installing a French drain and waterproofing to help direct water away from the very soft historic foundation material.  

Waterglass turns hazy reliably under two conditions:

1. If the surface is not absorbent such as on bricks that have dense glazes or that have been sealed previously
2.  If you do not take care to protect the work from drying out too quickly such as by exposing the work area to dry winds, direct sun, or hot temperatures in the first 12-18 hours after application.  Usually, the haze will appear within 15 minutes to an hour if conditions are not favorable for curing. 

Because of this, you should always do a mock-up first to ensure both absorbency and proper curing conditions. Less commonly but still a possibility, Waterglass can also turn hazy in the presence of acids, such as if the wall is acid-washed after pointing or if a recent acid washing or acidic cleaning product was not properly neutralized before applying the Waterglass.  If you notice Waterglass starting to haze, you can rinse it away with plentiful water in the first hour after application.

 If you just have a few hazy spots but the bulk of the wall looks good, it could be that you left some excess material on the wall in those spots.  This can be addressed by blotting up any standing excess Waterglass while its still damp with a clean absorbent cloth, or after curing, by scrubbing with a dry stiff bristle brush to remove any powdery material on the surface (but remember to never use metal brushes and always do small tests first to ensure your scrubbing isn’t damaging the surface).

No, never add water to Ecologic Potassium Silicate Paint or Ecologic Colorwash Stain.  Doing so can cause the components of the paints to separate and may result in splotchy, chalky, or hazy finishes and a lower service life.  If you need to dilute PSP or CWS, you should only ever do it with Waterglass.  PSP can only be diluted 20%, but CWS can be diluted up to 99%.  Keep in mind that diluting with Waterglass will change the color, especially in CWS where it will become more translucent and greyer.

Silane works by penetrating the brick and densifying the surface.  It also raises the pH of cementitious materials (like mortar) to help resist water and salt intrusion.  This can be great in certain situations, but is not necessarily something you need to do in all situations.  Whenever you make something denser or change the way water naturally moves, you can potentially cause deterioration especially on very soft masonry.  If the chimney has not had water problems and if the pointing is done well, silane may not be required. 

I always recommend being conservative with your interventions on a historic building–do the minimum necessary to keep the building preserved and don’t try to do more because every change can have unpredictable results.  Now, if water is an issue in that area, a silane can potentially be a good option, but no product, no matter how effective, is a “magic bullet” that solves all problems with no potential problems so you should always way the risks vs the costs when making decisions and see if you can eliminate water at its source rather than simply trying to stop it at its destination.