Stucco Over Historic Stone Buildings, FAQ Friday


Q: Why are beautiful stone farmhouses and other historic stone buildings covered with stucco? Is it for insulation? If the stucco over stone is an original and historically accurate detail, is it then OK to remove it to expose the stone and leave it that way? Will this enhance or deflate the value of the building in its authenticity?

A: Traditionally the only exposed stone is one with a gauged mortar joint. “Free stonework” are irregular pieces of stone shaped to fit with large, squared corner stones with alternating lengths used as borders. Ashlar work is varying sizes of cut blocks of stone that are laid in uniform coursing. You will sometimes see semi-coursed stonework on the front of a building and haphazard stone joinery on the sides and the back of the building. If the stone was shaped with tools used by masons it most likely was meant to be seen. You will often see remnants of the original external plaster or whitewash in the pours of the stone of the building which has already had the stucco removed to tip you off that the building was originally covered or coated and not exposed.

Fieldstones are stones picked up off the field when settling a property and preparing the ground for farming. They are laid up in “rubble work.” Some masons pronounce it “roobil” work. I think they are just repeating the accent of the old-timers. Rubble is junk. Fieldstone is just junk stone It is not dressed up in any way.

But the question remains, “Why did they cover the stone with exterior plaster?” Well, when you don’t gauge the joints and keep them tight the surface exposure to the elements is increased and accelerated the erosion of the pointing mortar. This may quickly deteriorate the bedding mortar and the integrity of the wall. It will at least aid in the transmission of water into the building. So, the same soft, punky mortar that was used for bedding was also used for exterior plaster, (stucco), and finished off with a shelter coat of whitewash. Whitewash is pure calcium carbonate lime and water. It was used as a waterproofer and protecting coat for both beauty and function. Whitewash could be thought of as a coating like an eggshell. It is soft, breathable and will protect the otherwise frail stucco render. Today the appearance of rubblework exposed is thought of as a thing of beauty. Historically fine stonework was squared and formal with straight, true and gauged joinery as the sign of high-end work. Really, it still is throughout the world, but “beauty is in the eye of the beholder.” If historic stone buildings where not plastered, (receiving an external stucco render), but instead received the inverted “v” joint to deflect the downward and angled drive of the rain, they usually were whitewashed right over top of the stone and joint in rubble work. When you don’t see the whitewash over the stone anymore it is because the acidity of a constant rainwater bath has loosened it and it has come off and was not renewed. More often than not it remains under the porch of houses and forebay areas of barns where it has been protected. Look closely in the pours of the stonework on the sides of the building and under the eaves or behind pent roof to see if remnants of the stucco or whitewash have remained. Another tell-tale sign that the building was originally stuccoed over the stone is that the widow trim remains proud to the stonework. If the trim comes out past the stonework at a thickness of 1-1’1/2″ past the stone, then that is indicative that the stone was covered with stucco to meet the outer edge of the wood trim.

The only insulation gained by exterior plaster is that of slowing a driving wind. Overall masonry is a poor insulator. 1940 and newer stucco may have had perlite incorporated into the mix to add an insulation element.

To correctly restore something would mean to put it back to its original design. For correct architectural restoration of a stucco over stone building means that the plaster should remain and be finished as it was originally. However, many people with unsound exterior plaster, which has lost its bond to the substrate or has cracks throughout it or has paint that is flaking, consider the removal of the offending stucco and coatings without replacing it but rather exposing, cleaning and repointing the stone. It is an option that will help mitigate the water infiltration problem. It is an option for overcoming the eyesore of flaking paint. It even increases the value of the building in many cases more than what the cost was to expose and repoint the stone. But my advise is to “just say no” when you have a formal exterior such as a building with a mansard roof. An exposed stone building which has been repointed and does not have the stucco or whitewash renewed should be reserved for a simple country farmhouse, outbuilding, or barn in my opinion. It may effect the value of the property in a negative way by removing historic details. A local historic appropriateness review board may not allow these modifications and a historical society may frown upon changing the unique and appropriate details originally found at the historic structure.

How Lime Mortar Traditionally Got it’s Color and How We Can Replicate this Today – FAQ by Randy Ruth

by Randy Ruth



Seven factors that can affect the color of lime mortar in no particular order of significance, Lime, aggregate, pigment, water content, rate of absorption, original surface texture and erosion. By its self, the color spectrum of lime can vary from bright white, light grey, slight pink or ochre colors. This color is dependent on a few factors such as the original stones chemical makeup and burning temperature. When an appropriate limestone is thoroughly burned (calcined) at a particular temperature to produce hydraulic or non-hydraulic quicklime and then hydrated to produce either a lime putty or dry lime hydrate, the result will be a white or off-white color. The first factor affecting the whiteness index of the lime will vary depending on the raw mineral impurities in the limestone. A limestone with a higher calcium content with all other factors aside will produce a whiter hydrated lime. If that same limestone is burned at a slightly higher temperature, the result will be a slightly grayer hydrated lime. Although, lime plays a role in the final color of a mortar, its significance today, when replicating a mortar joint is minimal, often due to the lack of availability or technical characteristics like Hydraulicity. Aggregate has a huge effect on the color of lime mortar. Historically aggregate for masonry mortar would be sourced from either local sand beds, found near creeks or rivers, or from the trimmings of stone on site and possibly brick pieces or dust as a pozzolanic additive.

The larger screenings of the aggregate play a role in the overall tone of the final mortar color but it is the fines that do most of the work. The smallest particles in the aggregate AKA fines will give the biggest impact on the final color. Brick dust, limekiln dust and clay impurities are pozzolanic fines that can be found accidentally and at times intentionally accompanying the aggregate. Today these impurities are almost never allowed into a replicate mortar mix, as the resulting technical data from such a mix design is often cost/time prohibitive for a project even if historically appropriate. As a result powdered pigments are often used today to achieve a particular mortar color. Just because pigments are predominantly used today in mortar mix designs, doesn’t mean that they weren’t used over 100 years ago. Colored mortar is an important design element in any building of today and yesteryear.

The types of pigments used in mortars have not changed all that much in past few hundred years. Iron oxide, carbon black, and natural ochre’s hold a solid footing in the industry today, each presenting their own limitations. It has been proven that carbon blacks can fade dramatically over a 30 year period in masonry mortar. Even though their tinting strength is very good, if not controlled carefully shades of grey can be very difficult to achieve. Natural ochre’s can produce wonderful colors and be very accurate when making accurate replicate mortars. The problem is in their tinting strength, and consistency in production on a large-scale. It may take above a 10% dosage of natural pigment to achieve the same color in a mortar using iron oxide pigments conforming to ASTM C979. Because of their durability, tinting strength and quality in production, iron oxides have been deemed the best pigment for coloring mortar on a large-scale. Even when using appropriate pigments at the correct concentration, water content in a mortar plays a big role in determining final color. Using the same exact mix in two batches and varying the water content by 10% will produce a significant change in color. From experience, I have noted that this problem is most evident when trying to achieve a red colored mortar. Light grey’s can also be problematic but are less evident and are usually deemed acceptable. That is why it important to measure all ingredients in a mix carefully to ensure consistency from batch to batch. The practice of mixing mortars consistently should carry over to pre-dampening of masonry units. By pre-dampening consistently as possible, the rate of absorption is controlled. This is a good practice just so mortar will not reach a flash set, and to control curing of the mortar which plays a role in the final color.


There is some debate on how a replicated mortar should look when not replacing all the mortar. Should it look new, with a smooth surface that stands out because of the way light reflects off two different surface textures? Alternatively, should it blend in with adjacent mortar joints? Personality, I believe in the latter. If a new mortar is inherently the same color at its core as the old historic mortar, than even though a slicked smooth replacement mortar will eventually blend in it can still distract the eye. A good repair for just about anything should be as seamless as possible. Besides, won’t a different texture erode differently, resulting in the continuation of a miss match over time? This brings me to erosion in mortar. As a mortar erodes, the color of the aggregate begins to come through. This color can sometimes throw off the human eyes perception of what is the color to be achieved when color matching. Someone explained this to me so well once that I must share. He had asked a room full of people what the color of foam on the head of a beer is. All replied white in color. We all got it wrong. The answer is amber like the color of the beer. This is because of the way light is reflected back to the human eye off a larger Surface area. Now that is in extreme case of a dark color turning lighter but the principle is still applicable to mortar. However, in most cases the rougher the color the darker a mortar is, and depending on who well the color of the sand is matched, you may just get a replacement mortar that will be seamless for generations. Ecologic™ Mortar

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Sandblasting Historic Buildings FAQ Friday


Q: Someone wants to sandblast my house. Should I let him sandblast?

A: Invasive cleaning methods such as sandblasting or high-pressure washing should be a questionable intervention upon historic fabric of any kind and should be considered carefully. Quartzite sand should never be the medium used on historic brick whose frail “fired skin” will be destroyed and thus expose a porous “salmon” center. Never sandblast sand upon any stone with intricate carvings or upon a terra cotta unit whose glazing would be irreversibly removed.

Other media is available such as ground up walnut shells, baking soda, diatomaceous clays, glass beads. Which media to be used is to be judged on their effectiveness using the most non-invasive method first. The non-invasive method is the one that works upon the historic substrate to lift only the undesirable contaminant. One could employ quartzite sand, (in varying gradation and at various p.s.i. pressure), when there is a sacrificial element allowance and need for aggressive cutting is desired such as in the case of removing tenaciously adhered Portland cement staining from a poor repointing job or from a cementicious whitewash when the substrate is common fieldstone that has no intricate carving.

Modern brick will lack the porous “salmon” center known to be the remaining condition of an historic brick fired in a down-draft kiln. Modern bricks are thoroughly fired in a tunnel kiln which results in more uniform densification throughout. But even modern brick will become “pitted” by the sharp sand action of a sandblaster. If any case where all the drawbacks and limitations are realized and anticipated beforehand still warrant the use of sand as the medium, this method is at your disposal if a test sample proves it is effective. However, be sure of this fact.

The surface area of masonry which is exposed to the elements is increased once sandblasted and micro-cracks may be introduced by any violently aggressive sandblasting especially by inexperienced operators. A final draw back in the use of this already unpopular method is that silica dust will be produced. At least a water mist used as a knockdown to dust must be engineered into the application. Water greatly reduces the dust when the sand comes out of the orifice and is pulverized into dust upon impact. A vacuum sandblaster is used when one must collect all of the contaminant such as paint along with the sand. Some municipalities do not allow sandblasting of any kind because of the negligence and misuse of the tool and because of the irreversible damage caused to historic structures. Not allowing sandblasting is generally a good idea since more damage is done than good over all. Should you let him sandblast? Most probably not.

Using Natural Hydraulic Lime in cold weather, FAQ Friday with Randy Ruth


As fall encroaches upon us with its cold weather, a question that is going to be popping up with more frequency is… Is it too cold to start or finish my project with NHL?

This may be one of the most difficult questions to answer, where the wrong answer can result in a lot of lost time and damage. The simple and safest answer is, do not perform work with NHL when temperatures will fall below 40 degrees Fahrenheit (including wind chill) for at minimum 7 days after placement of the mortar. Even following this model answer can result in frost damage of the mortar in some cases. So what is an applicator to do, to ensure that their work will last a reasonable amount of time? Well, care, caution, attention to detail, and patience is the answer.

If you can wait until spring to complete the project then you probably should to play it safe. If however, you absolutely need to complete the project before winter and do not mind playing with fire, then tenting and heating the scaffolding is an option. Tenting and heating can cost a lot of extra money in labor and fuel, so obviously make sure that there is enough money budgeted aside to warrant this approach.

By completely encasing the scaffolding envelope with heavy-duty plastic and being sure to affix the uppermost part of the plastic to either the roof or its eve, one can create a tight enclosed space for heating. When using a heater make sure that it is in a safe place and slightly raised off the ground. You should refer to any local building codes to make sure that you are in compliance and most importantly safe.

Of course, good masonry practices should not be skipped over just because the work is tented in and heated. Damp curing with burlap is still recommended and when repointing work is being executed good compaction of the mortar against the background mortar is still a must.

There are a few other tricks available to the applicator that can help prevent frost damage. One is the use of air-entrainment in the mortar. Careful dosing of an air-entrainer can help but not eliminate frost damage due to improper curing practices. Adding air-entrainment must be done with caution, as too much air in a mix will make the mortar weak and friable. When using proprietary admixtures, proper testing should be conducted to make sure that there are no adverse side effects .

Lowering the water content of the mix and increasing mixing time will help reduce the amount of water available to freeze without sacrificing too much workability. The use of warm mixing water, preheated sand as well as preheating the masonry units will help slow the development of frost. Winter accelerators associated with working with Portland cement mixes should not be used, such as calcium chloride or any nitrates. Depending on the properties of the surrounding masonry units NHL 5 can be appropriate to use in a masonry mortar to help withstand frost, due to its faster setting time.

When in doubt about working in possibly freezing conditions or allowing recently completed work to be exposed to freezing temperatures, you should probably trust your gut and call it a season. Dancing with the weather can be costly and should be avoided.

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