Sonoma Magnesite Mill

The Sonoma Magnesite Mill is shown in this photo mid way through its useful life.  Click photo for a more detailed image.


The detailed image has various locations in the main building and the right lean-to labeled.  The lean to at the front of the building was an addition after the main building was constructed in order to accommodate a second rotary kiln.  When the second rotary kiln was added, some of the equipment needed to be moved out of the way, necessitating the lean-to.  This series of photos shows the evolution of the mill.

This first photo dates to October 9, 1915 when it was printed in the Guerneville Times.  Photo provided courtesy of John C. Schubert.  Annotations on these photos are mine and are the best judgements on what individual components of the photos represent.  The first photo is just prior to beginning production or shortly after production began.  Note that the siding on the front wall is not complete.  I suspect it was left open to allow the rotary kiln and other equipment to be brought into the building.  The kiln is installed at this point as evidenced by the stack protruding from the top right of the rear gable of the roof.  Click the photo for a larger image.


This image is from a August 25, 1917 U.S. Geological survey.  It is a cropped and more detailed version of the first image.  I believe this is a construction photo of additions being made to accommodate a second rotary kiln.  It hasn’t been installed as there are only two large smoke stacks protruding from the roof.  I believe the right crushing area no longer has its exterior siding as it is being prepped for dismantling.  Addition of a second kiln would have bumped plant capacity to 60 tons a day.  It is likely that required an upgrade in the rough crushing capacity on the right side of the mill.

The shed on the front housed an interior siding for loading calcined ore on flat cars along with the fine crushing equipment.  The shed was needed in order to move equipment off the floor of the main structure to make room for the second kiln.  Click the picture for a more detailed image.


This the and fourth photos were taken in November 1923.  The third is from the collection of Jane Barry and the fourth from the collection of Hart Corbett and was provided courtesy of John C. Schubert.  The photographer is facing the left side of the building as viewed from the front.  The second kiln is clearly installed in this photo as evidenced by the stack on the front gable of the main building and the oil tank feeding the kiln through the wall.


The fourth photo was taken about the same time.  This image shows the siding that was constructed entering the front shed addition which provided a covered space to load calcined ore bags and barrels.  I believe the fine crushing equipment was also located in this room.  The other notable change is the timber structure to the right of the building which replaced the rough finishing shed in photos one and two.  This structure would have housed a large rough crusher like a Blake crusher.  The timber structure would have expanded the hopper capacity of the crusher, elevated it off the ground and probably also housed screening equipment that separated ore crushed to 2″ or less from oversized ore.  The ore passing the screen would have been routed to the kiln for calcination.  That failing the screen would have been run back through the crusher.  Click on the photo for a larger image.


This is the sole interior shot of the main building.  The shot looks down the interior rear gable taken from the left side of the building.  This shot is most likely to have been taken prior to installation of the second kiln.


The original kiln was located along the back wall of the main building  That means the front half of the building was initially available for other equipment involved in ore processing.

Lets see if we can figure out dimensions and materials for the structures.  I’ll rationalize assumptions as to dimensions.

Main Structure

Length – 70 feet in 1:1 scale.  The rotating kiln is 50′ long.  Both the cooling on the left side would have occurred in the main structure.  The fine crushing probably occurred in the left lean-to shed after the calcined ore had cooled.  The rough crushing on the right side apparently initially occurred under a lean-to shed in the earliest photo.  Later, the right lean-to was replaced with a timber crushing tower.

Width – 30 feet in 1:1 scale.  The kiln appears to take roughly 50% of the space on the back side of the building.

Wall Height – The top of the kiln hopper is even with the side wall.  If you look at the earliest photo above, a man appears to be pushing a wheel barrel along side the building.  From his size, I’m estimating the exterior doors to be 8′ high.  They would have been a minimum of 6’8″  That would have put the side walls at roughly 16′, a fairly standard dimension in construction.

Roof Pitch and Height – The roof pitch appears to be around 4:12, 4′ of rise for every 12′ of run,  That would make the peak 5′ above the side walls or 21′ off the ground.

Framing Timber – Side wall studs appear to be 4′ on center, possibly made from 3″ x 6″ timber.  Construction is much like a common pole barn today.  I suspect the wall lateral pieces are 2″ x 6″ nailed to the outside of the vertical timber, and are spaced every 4 feet vertically.

Roof Trusses – Are also spaced every 4′.  The laterial cords are probably made from 2″ x 8″ lumber.  The rafter portion would have been made from the same material.  A vertical cord supports the middle of the truss spans.  A diagonal support stretcher hits each vertical framing timber about 5′ from the ceiling and the lateral cords about 1/4 the way across the span.  I’m assuming the trusses are mirrored on the front side of the building.

Celestory – There is a celestory along the peak of the roof.  Presumably the purpose was to allow hot air and dust to be expelled.  It appears as though venting is supplied with sheet metal grates.

Wall coverings – are clearly ribbed metail siding.  It looks as though the same material was used on the roof.  The top 2′ appears to be made of a semi-transparent material that admits light.

Floor – I suspect the floor was made up of wood boards supported by wood beams. I suspect 2″x8″ flooring.  In the area underneath the kiln, the boards are protected from fire with two layers of brick.  While there probably wasn’t a basement, some provision will need to be made for transferring cooled calcined ore to the fine crusher for crushing.

Fine Crushing Lean-to Shed

If you look at the photos the left lean to extends more than 1/2 the depth of the structure.  It looks to be roughly square.  I’m estimating its dimensions to be 18′ by 18′.

Wall heights on the outside end would be 12′ increasing to 16′ by the time the roof intersected with the main structure.

Power House

It is hard to estimate the depth of the power house from the photos but I suspect it extended to the rear of the building.  Given the front lean-to is 18′ deep, the power house would have been 12′ deep.  The length appears to match the front lean-to so would be 18′

The shed roof appears to meet the main building just under its roof line so height at the rear of the power house would be just under 16′ with the height on the front side just under the main structure roof at the 12′ point.

Note that in the later photos a small shed has been added to the power house presumably to add to its capabilities.  That shed would appear to be 12′ by 12′ with a roof on the low end of 9′ rising to 10′ on the high end.

Rough Crushing Lean-To Shed

This shed appears on the right side of earliest two photos.  It appears to be similar in size to the fine crushing shed on the left with a similar roof line.  Fine crushing shed dimensions were estimated at 18′ by 18′.  Wall heights on the outside end were estimated to be 12′ increasing to 16′ by the time the roof intersected with the main structure.  Note that this shed was dismantled and replaced with a timber structure containing a larger crusher about the time the second rotary kiln was installed.

Timber Rough Crushing Structure

This structure appears in the last photo and is likely to have contained a large Blake Crusher and screening used to separate crushed ore that passed a 2″ screen.  Crushed ore passing the screen went to the kiln for calcinating.  Crushed ore that didn’t pass the screen was routed back to the crusher for crushing.  This appears to be a roofed structure with the top sides open as the hill can be seen through that portion go the structure.


Height appears to be close to the roof of the main structure plus the celestory or roughly 23′.  The top of the side walls up to the opening would be roughly 19′.  Timber braces are likely to be 4″x8″ or there about.  There are 5 timber braces per side and the structure appears to be roughly square.  Diagonal braces uses to prevent racking are  likely to be 2″ x 8″.

Hopper wall boards are also likely to be 2″x8″.  It is likely the hopper boards extend roughly 1/2 way down the structure.  The structure below the hopper boards appears to be somewhat open.  I suspect the 2″ screen would have been mounted diagonally with material failing the screen dropping off to the rear of the structure.  That passing through the screen would fall to a ramp mounted at a diagonal with the ore passing the screen dropping off in the direction of the rotary kiln.

I would estimate structure dimensions as 12′ x 12′.


My current inclination is to model the mill with the additions that occurred in around 1917 to 1918 (final mill image).  I’ll model with both kilns to deal with the production driven by World War I and an additional ore discovery.  I’m going to assume the timber rough crusher had arrived and was installed.

In order to display what is occurring inside, the main roof will be removable along with the roof to the power house and to the fine crushing area.  Sliding doors on the front shed addition will be modeled open for additional viewing.

Builders logs for construction of component parts of this mill setting will appear just below as the projects are activated.


Leave a Reply

Your email address will not be published. Required fields are marked *

You may use these HTML tags and attributes: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <s> <strike> <strong>