Attachment_3_-_Consultation.pdf

Christenot Mill Masonry Consultation. Sean B. Smyth, Stonemason.

Site Visit: August 7, 2022 Today’s Date: November 27, 2022

The old Christenot Mill site is an impressive and peaceful place to behold- a testament to good design and skilled trade work 150 years ago. The many and varied failing masonry sections are a testament to work needed to slow the ongoing process of the forest reclaiming its rubble.

Many patchwork masonry repairs have been carried out. Some of these repairs are passable, others are not. I must clarify the differences between modern mortar mixes and historic lime mortar construction. Upon reading the 1997 brief and reviewing photos from the last 25 years of repair I see much of the information is in the old reports, but the wrong set of mortar prescriptions were followed. Understandably, the nuances of historic mortars can be mind numbingly dull and are easy to miss without proper experience. Bear with me as I recap some of the basics and then move on to methods and cost estimates.

Christenot Mill was built with Natural Hydraulic Lime mortar and local sand. The lime was most likely produced by firing local limestone in nearby lime kilns. That limestone turned to lime would then have been slaked with water in a violent chemical reaction just to the point of creating a useable lime putty or powder, but not slaked fully to the point of becoming modern hydrated lime, also called builder’s lime. Natural Hydraulic Lime (nhl) has very different, almost living properties, compared to builder’s lime. Nhl, when mixed with the proper ratios of sand and water, requires days to cure under very specific conditions: damp, without freezing, without over exposure to sun and wind. The resulting joints and walls, properly cured, breathe with the seasons and the changes in weather. Water is absorbed and evaporated, slowly inhaled and exhaled through the joints. The joints are autogenous in nature- they self heal within reason due to the ongoing chemical curing that occurs with exposure to carbon dioxide. When a small crack occurs in the masonry wall, the newly exposed interior nhl mortar reacts to the carbon dioxide in the air and forms a new skin, preventing the crack from expanding deeper into the wall.

The properties of Portland Cement based mortars are much different. Portland cement mixed with builder’s lime and sand in various ratios yield different types (S,N,M,O,K) of inflexible mortars. Portland cement mortars are great for certain applications requiring compressive strength and resistance to moisture. Portland cement by nature makes the mortar inflexible. Portland cement was developed in Europe in the mid 1800’s and came to the U.S. between 1880 and 1900. Like many things, it was more prevalent on the coasts first and took a while to get inland. The overlap of portland cement and historic lime mortars took place in Montana in the 1900-1930 range. Christenot Mill was most certainly built without portland cement. When portland cement is added to repair mortars, the results can be catastrophic- whatever masonry sections receive portland cement mortars become inflexible, in the midst of other portions of walls that continue to flex and breathe with significant weather events, the changing seasons and ground tremors. If the Christenot Mill had been built out of soft brick, the “repairs” would have destroyed the structure. Stone is more forgiving. The use of portland cement in previous repair mixes increases the difficulty and skill level necessary moving forward with maintenance and repairs. Some of the portland repairs should remain in place, as removing those patches would cause extensive damage to the surrounding work.

Some should come out. Knowing which to remove and which to leave is a matter of nuanced on site experience during the work of repairs, not a matter of making a list. If the mill were to be rebuilt, all the portland mortar mixes should be removed and replaced with appropriate nhl mortar. My understanding of proposed future work on site is not a rebuild but maintenance to stop further decay and maintain the current state of standing walls.

Without a roof over the walls, the previous use of portland cement as a wall cap is passable. I do not suggest removing any of this capping mortar work. The major vertical running cracks that have been packed with portland cement should be left alone as well. The many patches smeared and tucked over original failing joints will need attention on a case by case basis- there are quite a few places where this inflexible mortar was applied without properly removing and cleaning the failing sections. This results in mortar simply masking internal issues that must be repaired. Most of these sections will need to be removed, cleaned out further back and replaced with appropriate nhl mortar. See first video and three photos. A half inch thick patch covers failing mortar joints six inches deep or deeper.

I recommend starting at the top of each wall section and working down. The lower, easily reached places have seen attention on past volunteer hitches. Much of that work will need to be redone. It is appropriate to start at the top of each section and work down. Scaffolding will need to be four frames high at each gable end. Four sections, properly planked and supported underneath with adjustable screwjack feet and dunnage, per OSHA specifications. I recommend using outriggers and walkplank to maximize working elevations. Natural Hydraulic Lime takes closer to seven days for initial curing under ideal conditions. This is key- nhl mortar must damp cure, not simply dry out. Also, it cannot freeze. In mountain settings this means the contractor must be prepared with concrete blankets and heat for cold nights as well as canvas, burlap and plastic coverings for hot, sunny and windy days. Order of operations must be well thought out as scaffolding cannot be moved until the upper most sections have cured enough to be left uncovered.

First, scaffolding is set. Then the portland cement mortar cap should be checked on each section of wall, repaired and replaced where missing. That is the only use of portland cement I consider acceptable on site, and this only because of past precedent and the lack of any future plan to rebuild the roof structure.

Second, all difficult higher elevation sections requiring scaffolding should be addressed.

Old mortar must be removed. The loose mortar is chased back to solid, then forced air is gently blown in the joints to remove dust. Then the walls are wetted down by hand pump sprayers using clean water. NO PRESSURE WASHERS should be used on these walls as the force of this water applied incorrectly could cause extreme failure. A note on safety here: the real nuance of a project like this is knowing what to take out, how much to take out, and the order of operations in removal and reapplication. The walls are thick, but unsupported structurally. Removing too much of one section before cleaning and replacing mortar could result in partial or total collapse of a section.

I recommend focusing on one room at a time. Section One top down, inside and outside (the office/vault room). Section Two top down inside and outside (the steam engine room). Section Three top down inside and outside (the barrel amalgamators/chilian crusher room). Each of these sections will take an average of 15-20 days skilled work at a minimum, plus cure time. I’m guessing the office inside/outside will take closer to twenty days plus cure time. The steam room between twenty and twenty five days, and the roller room end wall closer to fifteen days. It’s hard to call those numbers exactly because so much is unknown behind existing patches.

The mortar required to mix, tuck and cure could change in volume quite significantly from one patch and one failing section to the next. Two inches back versus six inches back triples the volume of mortar required. Easy math, but that is the asterisk on my estimations. Some flexibility should be built into the numbers, and I suggest starting with one section top to bottom rather than all three rooms at once.

After the walls are completely gone over, loose mortar and failing patches removed, cleaned, repointed from the top down and properly cured, attention should be paid to the back walls of the roller/crusher room. So much water has passed through those walls, washing away the mortar, that they appear to be dry stacked. They were not dry stacked. They do require cleaning, repointing and capping. These walls are the lowest ranking item on the masonry triage list because further blowouts won’t be as catastrophic as a wall failing. Was the proposed machine work to excavate and install drainage completed years back? If hydraulic pressure is still an issue each Spring I suggest incorporating weep holes into the repair work on this back wall. Weep holes low down in the walls allow water to flow through the base of the newly repointed walls.

Natural hydraulic lime mortar will breathe, but can still be damaged by excessive hydraulic pressure.

A note on contractor selection: The historic mason selected for this work must demonstrate an expert level of knowledge regarding natural hydraulic lime mortars and a work history including projects demonstrating this knowledge and practical application.

The contractor should know where to source nhl in the U.S., how to mix it, have a wide variety of repointing experience, and know how to properly cure nhl mortar in mountain settings. Most “restoration masons” I have met in the Mountain West have never worked with natural hydraulic lime mortars. The contractor should also be skilled in the use of a pneumatic hammer and chisel set for mortar removal rather than relying on grinders or high impact demolition hammers.

A local sand source is preferred- the nearer the site the better.

Access is difficult. Scaffolding should be brought up on the smallest possible trailer.

Mixing water must be relatively clean- ideally from the local creek, otherwise brought to the site via tank truck/trailer.

A First Aid Kit on site should include a portable eye wash station as nhl is caustic to the skin, mucous membranes and eyes.

I would start with 10-15 bags nhl and a yard of local sand per room (rough estimate).

NHL costs 75-100 dollars per 55# bag to get from source to site.

Scaffolding: rental, contractor provided, BLM supplied (preferred) Sand- will cost more to transport than the raw material itself. 3-5 one ton loads Labor is the main expense: 20 days skilled, 25 days skilled, 15 days skilled.

$45,000 labor, $10,000 supplies and transport. Rough estimate $50,000-$60,000 total.

Key for photos and videos on flash drive:

Depth of joint removal needed: three stills one video Original mortar in good condition: one video Failing mortar patch: one video Section one (office/vault room) rough estimation: three videos Acceptable portland cap: one photo Section three (barrel/crusher room): two videos showing various patches, one video describing nuance/timing of work, one video showing last priority.