Saturday, October 15, 2011

Brendan’s Mandolin Part II: Rosette

Above, you can see the two book-matched halves of Brendan’s Red Spruce soundboard being glued together. There are many fancy systems for this task but I prefer good old sash clamps- you can adjust the pressure to exactly where you want it to be.

Once the ‘board is glued, it’s flattened and taken down to around 4mm prior to inserting the rosette.

This is the first time that I‘ve used Red Spruce and I’m very impressed by its stiffness and glass-like tap-tone. The stiffness is especially important on this mandolin, as Brendan uses quite heavy strings and according to my calculations the extra down force exerted on the soundboard would be the equivalent to another 2.5kg pushing down on the bridge, compared to the load applied by the gauge of strings that I normally use.

Anyway- with the soundboard cleaned-up, it goes into my jig which allows me to route oval sound holes and channels for rosettes.

Once the channel is routed (and after a wee bit of fettling) rings of fine purfling go in. The outer ring is easy- you cut the purfling exactly to length and it’s squeezed into the channel and it forces itself against the edge, requiring no additional pressure. The inner ring is a bit trickier, so I use small pieces of spring bronze to hold the purfling in place whilst the glue dries.

After the purfling is taken down level with the soundboard, comes the fun part; cutting tiny diamond shapes out of turquoise! I had a few attempts at cutting the turquoise but it became clear that it was too thick, so I reduced the thickness from 3 to 1.5mm. A real waste, as the turquoise is sold by weight!

The turquoise is glued onto plywood, to support it and cut into 3.5mm strips. The diamond shapes are then cut off (easier said than done). Once the diamonds are cut they are then soaked in water, to dissolve the hide glue attaching the turquoise to the ply. As you can see I cut far too many; this allows me to select the best ones and also I suspect that Brendan may want a few of these in the fingerboard.

The dots and diamonds are positioned in the rosette’s slot (now that took awhile!) and glued in place with CA.

The next step is to fill in the gaps with “mastic”. Mastic used to be a mixture of ebony dust and hide glue which was commonly used to fill gaps around inlays. My mastic is a mixture of epoxy resin and cocobolo dust. This means that the rosette will match the back and sides and the burnt sienna colour will work much better with the turquoise than black. Of course, all of this is to Brendan’s specification and is the result of many emails and test-pieces; stuff like this doesn’t just happen!

The epoxy takes 24 hours to set and after some very careful cleaning-up,.....

Now is that pretty or what?


Anonymous Anonymous said...

Yes, that is great. Going to use a similar idea on my next mandolin

8:18 pm  
Blogger Unknown said...

Very nice work

10:26 pm  

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