Tournis (Mäusesteine) are circular stone discs hand-quarried from dense alpine gneiss or quartz schist, measuring 80–110 cm in diameter and balanced atop upright timber or stone stilts. By creating an insurmountable 20–30 cm horizontal overhang with beveled undercuts, they mechanically prevent rodents from entering elevated granaries. The unmortared gravity assembly supports 10–30 metric tons of deadweight while dampening alpine seismic tremors.
At a Glance: Key Takeaways
- Disc Petrology: Foliated Alpine Gneiss, Sericite Schist, or Granodiorite
- Standard Disc Diameter: 80 cm to 110 cm (typically 90 cm nominal)
- Disc Thickness: 8 cm to 15 cm with 45° beveled undercut perimeter
- Supporting Column: Squared or Octagonal Larch Heartwood Stilt (30–40 cm dia)
- Joint Philosophy: Dry unmortared gravity seat with lead/oak leveling shims
Petrological Selection and Quarrying of Alpine Gneiss
The mechanical efficacy of the rodent barrier in a Valais raccard begins with the petrology of the stone. Craftsmen did not harvest generic fieldstones; they searched high alpine moraines and scree slopes for specific formations of foliated alpine gneiss (such as the famed Arolla gneiss) or fine-grained quartz-sericite schist. These metamorphic rocks possess distinct parallel planes of mineral alignment (schistosity), allowing masons to split large, perfectly flat slabs of uniform thickness using simple iron wedges.
The stone requires exceptional compressive strength (exceeding 140 MPa) to resist crushing under decades of heavy grain loads and winter snowpacks. Furthermore, high quartz content endows the stone with near-zero porosity and high frost resistance. When subjected to the brutal alpine freeze-thaw cycles—where daytime meltwater freezes solid at night—the stone refuses to absorb water, preventing the spalling and delamination that would compromise its smooth overhang.
Geometric Mechanics of Rodent Isolation
The biological barrier created by the tournis relies on precise anatomical physics. Alpine house mice (Mus musculus) and black rats (Rattus rattus) are superb climbers capable of ascending rough vertical timber posts. However, their physical anatomy requires that their center of mass remain close to the vertical climbing surface. When a rodent reaches the top of a column, it encounters the horizontal plane of the stone disc projecting outward by 20 to 30 centimeters.
To bypass the stone, the rodent would need to invert its body, hanging upside-down by its claws from the stone face. Because master stonemasons dressed the underside of the stone with a fine chisel and cut the outer rim with a 45° downward chamfer, there are no micro-crevices or rough fissures for claws to grip. The animal’s gravitational vector pulls its hind claws away from the rock, causing it to fall cleanly back to the ground.
Structural Load Paths and Seismic Friction Joints
From a structural engineering standpoint, the tournis assembly is a masterclass in dry-joint gravity tectonics. A single raccard typically rests upon six to nine stilt columns, each bearing anywhere from two to four metric tons of deadweight. The vertical load passes from the heavy slate-clad roof, down through the interlocking corner-notched log walls, into the horizontal sill beams (sablières), across the gneiss discs, down the stilt columns, and into the sub-cellar bedrock foundations.
Crucially, there is no rigid mortar or mechanical rebar connecting the stone disc to either the timber column below or the beam above. The assembly is held together entirely by gravity and surface friction. In the seismically active Canton of Valais—which experiences periodic alpine tectonic tremors—this unmortared dry joint acts as a sacrificial base-isolation mechanism. Mild ground vibrations dissipate as micro-friction across the stone-wood interfaces rather than transmitting catastrophic racking forces into the timber crib above.
Maintenance, Leveling, and Modern Timber Column Restoration
Over centuries, alpine hillside creep and the slow decay of foundation stones can induce differential settlement across the stilt array. If a single column settles by even two or three centimeters, the rigid log crib above begins to twist, redistributing crushing loads onto adjacent tournis discs and causing them to crack.
Traditional millwrights and modern preservation carpenters correct this through a process known as re-shimming. Using heavy screw jacks or timber levers, the granary frame is gently lifted a fraction of an inch. Conservators inspect the stilt end-grain for dry rot or woodworm infestation, replace degraded birch-bark capillary pads, and insert hand-split sheets of lead or quartz slate shims to re-establish an immaculate, coplanar horizontal bearing plane across all stone discs.
Petrological and Mechanical Properties of Alpine Mouse Stones
Use this reference table to calculate exact physical dimensions, ratios, and specifications before purchasing:
| Rock Type | Compressive Strength | Cleavage / Workability | Moisture Absorption | Long-Term Durability |
|---|---|---|---|---|
| Arolla Gneiss (Granitic) | High (140–180 MPa) | Clean planar foliation; easy to split | Very Low (<0.3%) | Superior (>300 years) |
| Quartz-Sericite Schist | Moderate (90–120 MPa) | Thin laminations; prone to flaking | Low (0.6%) | Good (100–150 years) |
| Penninic Greenstone (Prasinite) | Very High (160–200 MPa) | Tough, irregular fracture; hard to dress | Extremely Low (<0.2%) | Exceptional (>400 years) |
| Alpine Sandstone (Flysch) | Low (40–60 MPa) | Easy to dress; poor weather resistance | High (3.5%) | Unsuitable; spalls in frost |
Practical Constraints
Every successful decoration project must account for physical and environmental constraints:
High Dynamic Snow Loading on Asymmetric Mountain Slopes
Unequal snow accumulation against uphill walls introduces horizontal lateral shear across unmortared tournis discs. Mitigation: Incorporate shallow stone mortises or concealed hardwood centering dowels between the stilt head and the stone disc.
Quarrying Heavy Gneiss Slabs in Remote Roadless Alpine Valleys
Transporting 200 kg stone slabs across steep glaciers and talus slopes without modern machinery was exceptionally hazardous. Mitigation: Historically cleaved slabs from glacial erratic boulders directly adjacent to the building site using plug-and-feather tools.
Execution Blueprint & Craftsmanship Sequence
Execute this professional sequence to achieve flawless results without backtracking:
Quarry Site Prospecting and Stone Cleaving
Locate fine-grained alpine gneiss erratic boulders on the mountainside; drill linear plug holes along natural foliation planes and split stone blanks using steel feathers and sledgehammers.
Circular Geometry Scribing and Rough Dressing
Scribe an 80–110 cm circle onto the split slab face using a compass rod; chisel away excess perimeter rock with a pointed masonry punch (pointerolle).
Undercut Chamfering and Honing
Chisel the outer 15 cm perimeter of the bottom face at a 45° upward angle toward the top edge, creating a sharp, claw-defying bevel that rodents cannot traverse.
Timber Stilt Framing and Mortising
Hew solid larch logs into octagonal posts (35 cm across flats); plane both end faces dead square and bore a 40 mm central locator hole in the top end.
Capillary Barrier Installation
Layer multiple sheets of resinous birch bark (Betula) or beaten lead flashing across the timber stilt top to prevent moisture wicking from the stone.
Tournis Hoisting and Center-of-Gravity Alignment
Hoist the heavy stone disc onto the stilt head using timber sheer-legs; center the stone precisely to ensure uniform distribution of granary deadweight.
Sill Beam Seating
Lower the massive squared larch sill beams (sablières) directly across the tops of the aligned tournis discs, allowing gravity to lock the unmortared assembly.
Periodic Level Verification and Shimming
Check stilt plumb and disc level with a plumb-bob and spirit level; adjust differential foundation settlement by tapping thin lead or slate shims beneath column bases.
Common Mistakes & Countermeasures
Avoid these costly errors that compromise aesthetics, ergonomics, and budget:
Mortaring the gneiss stone disc to the timber column with lime or cement mortar
Using sedimentary sandstone or soft slate that weathers rapidly
Allowing access ladders or stairs to touch the granary threshold permanently
Project Execution Checklist
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Frequently Asked Questions
Why are tournis discs almost always round instead of square?
Circular geometry maximizes structural efficiency while eliminating vulnerable 90° external corners where climbing rodents could find bridging footholds. Furthermore, round discs distribute heavy vertical compressive loads uniformly across the timber stilt head.
What keeps the unmortared raccard from blowing off the stone discs during alpine gales?
The immense deadweight of the structure. A fully loaded raccard containing winter rye sheaves and stone slate roofing weighs between 15 and 35 metric tons. The colossal frictional resistance between the heavy larch sill beams and the rough gneiss discs easily resists lateral wind shear.
How do artisans prevent the stone disc from splitting under the point load of the granary sill?
Joiners hew the underside of the larch sill beams completely flat and insert compressible bedding pads—such as birch bark, cleft oak shims, or thin sheet lead—between the wood and the stone disc, spreading the concentrated load evenly over the stone surface.
