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

Designing a Jacobean Fireplace continued

Mantlepiece, left side

A half day was spent taking our sketches and fleshes out the details in full scale technical drawings. A review with Professor Warren resulted in some minor detail changes. I'm used to thinking of finer plaster details and stone often needs to be more robust to physically hold up to the mallet and chisel.

The redlines led to a minor cascade effect that ended up in another half day of redrawing. Better to work it out in pencil than face an issue in the stone.

Jamb profile and plinth

One of the things I've enjoyed learning in the process is the fireplace specific nomenclature. I have a fairly broad base of architectural vocabulary that didn't necessarily apply and found myself at a loss at what to call things. I believe I now have it sorted out and the pieces are labeled correctly, at least in the image captions.

Mantleshelf

Sam and I have decided to leave the spandrel and frieze areas of the mantlepiece and the concave profile of the mantleshelf solid for now, focusing on plain profile work. That way if a client is interested in the surround we can customize ornamentation for them. That won't preclude us from drawing some conceptual sketches later in the semester.

Our colleague Charles is designing an entire fireplace including the surround and firebox (and the hearth I would imagine) in brick masonry. I'm yet to discuss the details with him but it appears he is designing the surround in rubbed and gauged brick with an ovolo pattern in the mantleshelf. Exciting stuff!

Contributed by Patrick Webb

The Gods of Rock: Pluto and Vulcan

Human beings have been working stone for a long time. It seems as though physically our hands and bodies are perfectly adapted for the task. Likewise, the ability to conceptually form stone into tools, hunting implements and structures might well be hardwired into our DNA. Such an enduring material, much of what is gleaned of early civilisation in the field of anthropology has quite literally been recorded in stone. This post initiates a three part series to shed a bit of light on the types of stones that generations of masons and carvers have found most useful. 

We Have Ignition!

We'll begin with what are called primary rocks*, so classified because they are the direct result of the original consolidation of minerals into stone. The common name for this group of stone is Igneous, derived from the Latin "Ignis" meaning "fire". That name is a strong indication of the origin of Igneous rocks, deep under the earth's surface in the lower crust or even the mantle. Oxidized silicon compounds (silicas, SiO2 or silicates, SiO₄) under extreme heat and pressure form and mix  into a liquid, molten state known as magma. How that molten magma cools distinguishes the two major sub-classifications of igneous rock.

Pluto - God of the Underworld...

and Riches

Most folks know Pluto as the counterpart of the Greek Hades, the Roman god of the underworld. However, the Romans had a more positive view of Pluto also considering him the god of wealth, a giver of abundant riches. This was in no small measure due to the mineral wealth the Romans were particularly adept at unearthing from mining operations. 

Plutonic rocks are Igneous rocks that have specifically cooled, underground within the earth's crust, surrounded by other rocks. The other rocks act as a kind of insulation. As a result Plutonic rocks cool very slowly. This allows time for large and long crystal formation, resulting in a coarser textured stone. Plutonic rocks are also classified as Intrusive rocks because the magma typically "intrudes" or fills horizontal or vertical strata known as "sills" and "dikes" respectively. Larger cooled magma chambers can produce "bathotliths", huge massifs that can account for entire mountain ranges.

The most common form of Plutonic stone used for masonry is granite. Although coarse, brittle and hard to work, granite is a beautiful stone that is exceedingly durable. Because it is hard to achieve fine detail it is best reserved for large scale, monumental work when carved.

Vulcan - God of Fire...

and Volcanoes

What is not commonly known is that Vulcan was also the god of metalworking specifically and crafts in general. Most often in ancient sculpture you'll find him hammer in hand surrounded by billowing fire and smoke. The stone temples of the gods were attributed to the hands of Vulcan. 

Volcanic rocks are also Igneous rocks; however, volcanic rocks are formed from lava, magma that has been discharged from underground into the sea or in the open air. Therefore, volcanic rocks are classified as Extrusive rocks as they are "extruded" as lava from underground magma tubes onto the surface where they rapidly cool. There are many varieties of useful volcanic rock for masonry and carving. Basalt in particular shares the density and durability of granite. Yet, because of the rapid cooling, the crystalline structure basalt is much smaller and finer allowing for greater detail in carving. The Giant's Causeway on the coast of Ireland is an amazing example of a volcanic basalt formation popularized by rock gods Led Zeppelin on the iconic cover of their Houses of the Holy album.

Next in the series we'll consider my personal favourite for carving, sedimentary stone.

*The terms "rock" and "stone" are used interchangeably in this post. Some consider "rock" to denote the natural form whereas "stone" is distinguished by or set aside for human alteration.

Contributed by Patrick Webb