What is a vlinder?
A vlinder is a timber board sawn to a specific curve and fixed to the centering used to build an elliptical arch. In Dutch it is called a vlinder (literally: butterfly). While laying, the bricklayer stretches the line over the curve of the board. The line then shows the direction of the joint at each brick, so that every joint stands square to the face of the arch.
How does it work?
With a round arch it is simple: all joints radiate from one centre, and a line from that point always gives the correct joint direction. An elliptical arch has no single fixed centre. The line square to the curve keeps changing direction and crosses the major axis of the ellipse at a different point each time.
The shape of the vlinder is therefore not arbitrary. It is set out as the evolute of the ellipse: the curve to which all normals of the ellipse are tangent. If you pull the line tight along the board to a point on the arch, it coincides exactly with the normal at that point.
Application
The vlinder is used for brick elliptical arches over doors, gateways and windows, and in the restoration of historic arched brickwork. The carpenter makes it together with the centering; the bricklayer uses it as a guide. Note that the Dutch verb vlinderen, power floating a concrete floor, is an entirely different subject.
Tips
- Set out full size: draw the ellipse and its evolute full size on a board or paper first, then transfer the shape to the timber.
- Fix it firmly: the vlinder must be secure on the centering. If it shifts, every joint will run askew.
- Keep the line taut: a slack line will not follow the curve and gives the wrong joint direction.
- Check the joints: because the curvature of an ellipse changes along the arch, the taper of the joints changes too. Check each joint with the line before the mortar stiffens.
Related terms
- Arch centering
- Centering segment
- Basket arch
- Round arch
- Bricklaying
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