Star of David, Merkabah, Duo-Tetrahedra

A tetrahedron is the only regular polyhedron that is its own dual. If a corner is placed at the center of a face on a polyhedron, you get a dual.

For example, take a cube. Put a corner at the center of each face of a cube, and you get an octahedron, not another cube.

If you place the corners at the centers of the faces of a tetrahedron, you get another tetrahedron.  Two tetrahedral superimposed on each other form a curious and notable structure, variously known as a hyperbolic paraboloid, or duo-tet, or a merkabah.

A merkabah is the shape of the chariot by which the prophet Isaiah ascended to heaven. It is supposed to provide a more full realization of the existence of God. Powerful stuff.

If we look at the surfaces between two edges of superimposed duals, they are a least energy surface. Soap bubbles fit these surfaces, and soap bubbles are an accurate representation of least energy (tension vs. strength) surfaces. These least energy surfaces are also as easy as twisted sidewalks in concrete.

Screeding is a method used to spread concrete evenly within a form. Two guide bars are used to direct the screed bar, which is drawn across the concrete in a reciprocating motion, evenly distributing the concrete. Normally, for a flat sidewalk, screed bars would be parallel.

If the screed bars turn through ninety degrees rotation over their unit length, then the edges of the screeded shape assemble into a hyperbolic parabaloid, or duo-tet, or merkabah.

A mason's trick

Masons have several "tricks" up their sleeves that they will use in the course of doing work.

One of these tricks is pretty sneaky and rather clever.  It's about insuring that they get paid for their work.

One of the most common jobs a mason is hired for is to build chimneys.  Anyone who has ever worked in construction -as a contractor- probably has their own tales about not being paid by the homeowner, or the main contractor, if you work as a subcontractor.  Here's a trick masons use to make sure they get paid when they build a chimney.

Toward the top of the chimney, a piece of plate glass is placed in the mortar, between bricks.  The plate glass is located so that it obstructs the air flow in the chimney.  If someone tries to start a fire, the chimney won't draw at all, and the house will fill with smoke.  If the homeowner looks up the chimney, they will see the sky, and will wonder "why doesn't this chimney work?"  Now if they haven't paid the mason, he or she can say:  "Pay me and I'll fix it."  After being paid, the mason then breaks the glass and the chimney works fine.

Just a method to ensure payment.   But if you upset the mason while they're working, you might end up with this:

Building a kiln

Kilns and furnaces generally run on one of two methods. First is convection, second is forced air.


Convection relies on hot air rising, and needs a tall chimney to induce convection (venturi effect) to make it work.

Forced air relies on a powered fan to force exhaust gas out of the kiln or furnace, no tall chimney is required.

I am currently attempting to convert a former gas kiln to a forced air wood kiln. Wood kilns are never forced air, not in my experience. They always have a tall chimney, and take a long time to fire (around 3 days, stoking every 15 min. or so).  Here's the kiln when it was operated with gas burners.  It uses the triangular interlocking block system I've been describing on this blog.

The firebox is an inclined catenary arch. Inner arch is recycled kiln brick, outer arch is recycled red brick. I used refractory mortar inside and regular mortar outside.

I hope it fires faster than a regular venturi driven tall-chimneyed boring-ass kiln. Yes, I said boring ass. Ever fire a wood kiln? Pretty damned boring.

I plan to put the blower at the bottom, and a door for stoking wood on top.  I'll do another entry on this kiln when it is completed and firing.

Modeled Sphere

My business partner Mike Wong made this drawing of an assembled sphere.  This is very close to what a water storage tank will look like.  Concrete blocks, around 3.5 inches thick, sphere about 9 feet in diameter, holds about 2,900 gallons. It is shown with an opening at the top.  Easy to assemble, very strong below ground.    Click on the image to see it better.