Random Rubble Masonry

 
This summer, the partners and sponsors of Projekt CHARME have invited two stone carving students and myself from the American College of the Building Arts in Charleston, SC to learn and practise the craft of random rubble masonry in the tradition of the Saxony-Anhalt region of Germany. All of us having previous experience in coursed masonry in stone and brick, so we're expected to become quick learners of random rubble masonry, ambitiously entrusted with a project to re-erect two walls of a 19th century orangerie in the "Landschaft" or public nature park of Althaldensleben. The walls incorporates quoins, windows, door and cornice elements as well.

Building in Stone

Architecture utilizing this tradition of random rubble masonry is exceedingly durable and to my eyes conveys  a rustic sublimity. Several structures in Hundisburg including two medieval towers, the local church as well as the fortification walls of the old "Burg" or citadel how incorporated into the present "Schloss", castle date back 800 years. All of these charming, hand built structures are still in continuous use and fine condition with minimal maintenance requirements.

Neither was stone masonry reserved strictly for the church or castle. The entire town of Hundisburg below and the neighboring Haldensleben typically have ground floors of random rubble masonry, above which you'll find timber framing with lime plaster rendered stone infill on the first and sometimes second floors. This method of construction, also known as half timbering, keeps the oak timbers elevated and thus protected from rot. Many of the homes have the name of the "Holzman" or timber framer and date of construction carved directly into the wood. Apparently there was a building boom here in the early 17th century with many residences dating from that period.



Rubble and Randomness

As the name might imply, the stones used are rubble indeed. "Bruchstein" is the name used locally, literally meaning broken stone. And you can see why! The stones are extremely hard, practically impossible to dress or shape by hand tools. I've been informed that they are igneous stones, clearly of the "plutonic" or extrusive type, having cooled rapidly they manifest no crystallization. That does not diminish their density or hardness, however. The most effective way to get them down to usable sizes in the past was to "break" them against each other.

The rubble is random as can be with a gradation of sizes or small boulders to small flat stones one might skip across a lake. All of these come in a variety of shapes and sizes that must fit together like a giant jigsaw puzzle.

Our particular stones come from the Schloss Hundisburg grounds of which the Lanschaft was formerly a part. They in turn were originally quarried from a nearby hill. The quarry long abandoned has filled up into a scenic little lake that serves as a pleasant public summer destination for the locals.


Mise en Œuvre

Dry stack or mortarless random rubble masonry is a big tradition in Scotland and many other places.
Nevertheless, in the Saxony-Anhalt, regional German tradition mortar is used. Traditionally is was always lime mortars; however, on our project the lime is gauged with cement to give an early setting and corresponding compressive strength so that we can work a little faster. As the Bruchstein has a higher degree of impermeability and compressive strength than the blended mortar, neither I nor the local "Steinmetz" or masons see any long term issue with the change in formula.

There are some differences in working with Bruchstein than in my experience. The most obvious is the random nature of the placement of the stones. Although we continue to string lines and use levels to assure the walls are plumb, I have had to forcibly resist the tendency to seek out stones that would result in a regular "coursing" or horizontal banding that is typical of the English and American traditions. Just to imagine how one piece of rubble could fit into another was at first a real challenge. The first week I was practically blind, very slow at selecting pieces but my vision has rapidly sharpened since. One truly has to let oneself go, allowing the stones to arrange themselves in an irregular tapestry. The single conscious decision that you must keep in mind is to make sure you are consistently using stones oriented to tie back into the middle of the wall. If not you might end up with essentially a veneer of stones at the face that would result in long term structural instability.

Another adjustment I've had to accustom myself to was not pre-wetting the stones beforehand. This is typically done with most stone types so that the mortar achieves a better bond. The Bruchstein is so impervious that even dampening them will result in your stones "swimming", sliding out of position. A stiff mortar we've learned is the best mix.

In addition to Bruchstein  we are incorporating "Braunstein", a rosy coloured brownstone. The Braunstein is comparatively much softer and workable with mallet and chisel. Students last year dressed several pieces into orthotopes or extruded rectangular units, edges tooled and centres punched to serves as both structural and decorative quoining. Additionally, we're utilizing odds and ends found in the castle boneyard of tooled "Sandstein" the local yellowish sandstone to cobble together a door and couple of windows. Again, students last year carved a couple of pieces needed to complete the ensemble. The German tradition of tooling is much rougher than what I'm accustomed to from the English tradition. However, I'm quite impressed with it as it reads very well from a distance and allows more room for error, increasing the speed of working significantly.

At the time of this writing we're about half way into our stay and likewise about at the midpoint of our project. The door and windows are taking shape, the castle masons have been impressed with us, now providing us every support and the local folks walking through the park have warmed up as well, taking the time to converse in my broken but improving German. It feels good to know that there is still so much to learn, I'm lucky enough to have the opportunity to do so and work with brother craftsmen who despite language barriers are generous enough to share.

Contributed by Patrick Webb