How to Make Curved Glazing Bars archive

Friday 12 February 2010

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The specification was a network of fine wood, surrounding and supporting shaped glass panes in a glazed cabinet door. Small offsets between adjacent glass facets subtly break up wide reflections, enhancing the appearance.

This kind of work calls for care with handling the complementary properties of brittle glass and thin, pliable wood. It requires moderate precision but equally benefits from maintaining flexibility in the woodwork, hence the choice of a hot-bent laminated structure.

My aim was to keep the glazing bars slim and elegant, with strength provided by their depth and reinforced by the grain direction. I used American black walnut (Juglans nigra) throughout, with PVA adhesive to bond the laminates, then thin picture-framing glass and hot hide glue to secure the rear beading, once the glass was fitted.

Design choices

The geometric flowing pattern was aimed at both aesthetic objectives and practical construction. The design is based on simple arcs and parallel tangent lines. The glass contacts the wood only in long grain so there should be no wood movement problems.

I started by exploring some optional patterns and ended up with this variation on a theme that echoes the figuring on the faces of the walnut boards selected for the cabinet carcass. In principle you might design and construct any shape of wooden frame or perhaps one similar to those made in leaded glass. In that case the wooden bars would have needed to be thicker to accommodate angled joints with adequate support for the panes.

Producing the layout on a computer makes it easy to adjust for rebates and clearances. If you have access to a large-format printer this is all well and good, otherwise you will need to print the design in sections as I did, then gum them together to produce a full-sized pattern or rod and the templates.

Alternatively produce the design with a ruler and compass or with French curves, then either trace or photocopy the templates from the master.

Templates

Three well-matched copies of the finished pattern are needed, one to make a rod on thin board used for laying out the components and two to make two sets of templates.

Using a craft knife, I cut a set of thin card templates for the glasswork. A heavier set of templates is also needed to act as formers or moulds for shaping the laminated glazing bars while their glue sets.

I fitted the bandsaw with a narrow blade to cut the curves from 18mm-thick MDF, then smoothed them on a belt sander before brushing the edges with furniture wax paste to act as a releasing agent. I carefully rubbed it well into the MDF to remove any surplus that could contaminate the glue.

Suitable frame

The whole project is made from 25mm walnut. I planed and selected boards with coherent figuring, then thicknessed them down to 20mm for the rigid rectangular support frame around the delicate glass and glazing bars.

The frame has a conventional mortice and tenon construction, sized to fit a small walnut wall cabinet.

Internally the frame is rebated with a 3mm lip to secure the outer edges of the glass panes. The rebate was sized to accept the outer edges of the full-sized rod as a clearance fit.

While preparing the frame it is best to plane and adjust its outside edges to fit the cabinet opening as well as cutting any external rebates for hinges and catches - processes that would risk damaging the finished door if they were left until the glass was finally fitted.

Shaping curves

Each element of the finished glazing bars - each vertical muntin and each arched rail - is 6mm thick. This width is built up by laminating four layers or lamina, each one being 1.5mm thick. Two central layers are full depth front to back and these form the ribs of each glazing bar, providing most of its strength.

Extra pieces of curved walnut are cut into narrow strips, one set forming a rebate in the lamination to hold the glass in place at the front and another set providing loose beading to secure it in place from behind.

The bandsaw must be fitted with a fine-toothed blade and adjusted so the fence is truly vertical, with a 1.5mm gap against the inner teeth.

Thin strips of walnut are cut on the bandsaw from the edge of an 18mm-thick board, the edge being planed smooth before each lamina is sawn from it. This means that each lamina will have one side sawn and the other planed smooth. These two surface types glue together to provide an even thickness provided the saw marks are fine.

Violin ribs

Laminating thin strips of wood with glue then pressing them between matching moulds is an effective way of pulling them into shape. However, there is always a slight uncertainty over the amount of spring-back to allow for after the glue has set. There will also be trapped tension in wood formed this way.

A technique used by violin makers to curve the ribs at the sides of their instruments is to pull them over a hot bending iron. Unlike steaming, this only works on thin sections, while the temperature is higher at around 140 degrees C. The hot-bending process is quick, with a high degree of manual control, and it does not raise the grain or increase the moisture content of the wood.

I fired up the workshop stove and used the enamelled chimney pipe as a bending iron. Unlike violin ribs that are C-shaped, the curves for this project are sections of a circle which, having a regular radius, are comparatively easy to form.

The process of hot bending is easiest to follow by watching so I have put a short video clip of this stage on the internet at www.furnituremaking.org

Laminating

Overall, the laminating technique in this example has a total of 24 individually shaped lamina, each 1.5mm thick. At every point on the glazing bars there are four lamina bonded side-by-side, hence a total thickness of 6mm.

The vertical muntins are curved over to produce arched rails at the top where the two central lamina part company to sweep in opposite directions. At this point the upper pairs of lamina are tapered by paring them with a sharp chisel so they meet cleanly on the inside cusp of the two arches.

The laminated glazing bars are glued with water-resistant PVA and reassembled in bundles before the MDF moulds are fitted around each bundle. The moulds are then clamped to apply firm uniform pressure.

Neat edges

The edges of the laminations must be kept in tight register while the glue sets otherwise the rebate lip holding the glass will not be level. Even so there will be some unevenness and glue will squeeze out and need planing off. Glue on the rebate edge needs to be removed with a knife before it sets too hard.

The option of shaped moulding on the glazing bars is a matter of taste, but it also discourages splintering on the curved arises and reduces the apparent thickness.

Traditionally, moulding would be applied at this stage with a scratch stock. Alternatively you can use a router table to apply a simple chamfer to the front edge.

Fitting bars

I used two elementary types of joinery in making this frame - six mortice and tenon joints around the edge and a single crossed halving joint in the middle. All these miniature joints are cut the conventional way with a fine dovetail saw and chisel. Being so small they do not take long.

The components are matched up, the tenon shoulders cut and the crossed halving joint is positioned using the pattern of the rod as an alignment aid.

Mortices are chopped on the inside edges of the frame, so the glazing bars must finally be finally sprung into place. I made the mortice sockets slightly over depth from front to rear to allow the tenons to be fed into them at an angle.

A wedge is then fitted in the excess socket depth behind the tenon to secure the end of the glazing bar.

Shaping panes

With the glazing bars fitted in the frame we need to recheck that the card templates fit in their individual apertures. If necessary the card can be trimmed at the edges to avoid catching. The card templates will be used as patterns to score then snap the thin glass panes to shape.

The glass-holding rebate on the glazing bars in front of the glass provides a lip of only 1.5mm, so clearances are tight to avoid gaps.

Sheets of picture-framing glass that have been recently manufactured are relatively easy to cut using a good-quality scoring tool with a lubricated tungsten wheel. This is even true when the template has gently curved lines.

Each card is placed beneath the glass as a guide for the scoring tool to follow, then, without delay, pressure is applied to the overhanging edge to make a clean snap.

There are pros and cons to cutting your own glass. It gives you more control over the quality of the job and makes it easier to remake any poorly fitting panes second time around. However, it does take practice to apply light, even pressure to the scoring tool without the wheel skidding or digging in and to make a clean snap.

If you have no experience with glass cutting, expect some misshapen pieces and breakages along the way, or it may be better to give the task to an experienced picture framer.

You may need to spring the glazing bars apart slightly to let the curved glass panes ease into their individually shaped apertures.

Beading

Hide glue, with its grab-while-you-wait property, is the ideal adhesive for fixing beading strips to secure the glass panes in their frame. On the inside, the glass is beaded in place with loose strips of walnut, sawn and shaped in the same way as the lamina, then cut to width, edge planed and fixed with hide glue.

Each beading strip is sprung into position to secure itself firmly against each glazing bar. To keep it tight, the length of the bead must be cut to the maximum to fit in the arched opening, so a bit of trial and error with spare materials is needed before applying the glue.

The finish

The finish of the front portion of the glazing bars can be completed before the glass is inserted, while the rear beading can be finished on its outer surfaces but will require further work after it is hide-glued into place. Any surplus hide glue that squeezes out onto the glass must be peeled off while still in the firm rubbery phase. Hide glue will eventually adhere very strongly to glass as well as to wood so the excess needs to be removed while it is still soft enough.

There are simplified ways of simulating the effect of glazing bars but the technique described here is real as well as being practical. A laminated structure can also survive the mishap of an occasionally slammed cabinet door. And if push comes to shove - hide-glued structures can be repaired.


Woodworkers Institute

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John Bullar , mastercraftsman , glazing bars , Georgian