It has been a long journey for us to come back to plaster for masonry heaters. The first two heaters that we built after we established our company we finished with clay plaster. Both of these heaters experienced cracking on their skin which led us to shy away from plastered finishes and into a body of work for over 10 years with brick and stone finished heaters. We are excited to get back into the modern look of plastered heaters and find that lime has a lot to offer in its combination of base white color and properties for application. We are interested in digging into the most up to date research and experience.
We are very lucky to have the makers of the above heater, André and Jean-David Morneau, with us in this forum under the user name @Cooperative_L_esprit. You can see a brief resumé of their plastered heaters in a design article we published entitled “The Inspiring Plastered Heaters of L’Esprit du Lieu”.
We believe that there are several layers of contributing factors to a successful plastered finish for masonry heaters.
Adequate considerations for expansion within the heater construction, including an expansion joint between a “core” and “facing”.
A good, strong base coat…
Some years ago, William Davenport of what was then Turtlerock Masonry Heat (famous for its slogan “Craft Not Dead”) published an a fondly referred to as “Shinerology” which reported on a process of using surface bonding cement and then embedding fiberglass mesh into the first layer of plaster. We learned from Jeremiah Church who previously worked with William of the technique of mixing thinset and regular mortar for the first coat and embedding the fiberglass mesh into this first coat.
An intermediary coat?
A nicely formulated finish plaster.
Here is a photo of a masonry heater project we are currently working on that we are looking forward to plastering:
Note that we have already deviated from William’s article in that we have chosen to use a type S cement mortar simply because it is readily available and we enjoy the laying flexibility the larger joints offer instead of working more precisely with thinset (and hadn’t re-read William’s article). It has been a very expedient process for us to lay up this facing and we are interested in the economy this process presents for keeping masonry heater budgets affordable.
We understand that the thinset/cement mortar first coat with the fiberglass mesh embedded makes for an excellent first coat but have some concerns about the use of a polymerized mortar like thinset and the off-gassing possibilities when exposed to unusually high temperatures. We understand that there are a good amount of builders with a decent number of these units in the field so we are interested in evaluating whether this is a valid concern. The elasticity of the polymers in thinset would certainly be an advantage in providing an amount of flexibility to the final skin that might resist cracks.
What do you think @Cooperative_L_esprit? What do others think? I am hoping to also invite others with experience to shed light on this topic.