Renewing a cork joint.

Read all of this first and prepare your tools and materials.

Woodwind repair materials can be obtained from Wind Craft Ltd. (Link)

Clean away all old cork, grease and glue residue. Use a knife blade and scrape the wood clean. It does not matter if you remove some varnish.

Measure the thickness of the original cork. (Use callipers if you have them.) 1.5mm is usually suitable if you are uncertain. (See the note at the bottom of this page for help with the measurement.)

Select a suitable piece of cork and cut it to fit the groove using a very sharp blade and a steel rule. Trim it to length carefully, slightly too long is better than too short. Cut the end clean and square. A good butt joint is entirely satisfactory.

Mix a small quantity of epoxy (preferably quick setting) on a scrap of waste card. (A pea sized total mix is enough.)

Apply a thin layer of the epoxy glue to the groove in the wood and the butt end of the cork. I find a large saxophone spring ideal. A large darning needle is a domestic substitute.

I have recently started using 'Gorilla' glue for small repair jobs. It is expensive but economical in use as no mixing is needed. Read the instructions carefully! This polyurathane glue cures in the presence of water and foams, so that it penetrates the work. It has very good adhesion to synthetic cork. I find that a small trail of glue, straight out of the bottle, about 1mm. across will spread over the usual joint area. You can always add a bit more if necessary, but don't over do it. Damp the inner face of the cork, and one end. Don't forget to apply glue to the other end of the cork strip.

Place the cork round the joint groove and lightly bind it into place with some thickish thread. You will need about 2 metres of thread for a treble. Button thread is about right. Make very sure that the ends of the cork are butted together. Work quickly.

Leave the joint to set (at least an hour, but see the instructions on the glue).

Remove the thread and try the fit. If it is too tight carefully sand the cork down with glass paper wrapped round a square cross section stick. Do not mark the wood. Do not apply grease if you anticipate sanding afterwards.

Note, on cork thickness etc.

Natural cork becomes much more flexible and manageable if it is hammered before use. You can do this to alter the thickness. Use many light blows on the pre-cut strip. It will grow in width and shrink in thickness. Use the distortion of its shape as a guide to your hammering and try to keep it a true rectangle. Re-trim when the thickness is right.

This same technique works very well for the synthetic cork substitute.

Glue the cork onto the instrument fairly quickly after hammering otherwise it will creep back to its original thickness.

If you have callipers you can judge the required cork thickness by halving the difference in diameter of the two parts of the joint and choosing cork the next half millimeter size larger.

Comments...

If the tenon (the plug in part) is cracked, you can first bind the wood with fine wire, cotton or polyester thread and glue before hiding it with cork. (Cut away the ends after the glue sets, there should be no knots.) You can also hide a packing layer of paper under the cork if you need the small extra thickness.

I far prefer a vegetable grease to vaseline (petroleum jelly) as a cork lubricant. Clear or white lipstick works well too. Joint grease for orchestral woodwind is entirely suitable and much better than the concoction provided by most recorder makers.

If you do not have callipers to measure the diameter there is a better way than by holding a ruler across the end. Cut a strip of paper and wind it round the part you want to measure. Don't line the edge up pefectly at the overlap. At some place where the strip overlaps itself make a neat mark with a sharp pencil across the paper from the upper part to the lower part. Unwind the paper and measure the distance between the two marks with a ruler, and note it down. Do you remember the maths you did at school, but have never used? Forgive me... all you have to do now is to divide your measurement by π ('pi'). 3.1412 will do if you have a basic calculator without it available as a constant. If you are working with pencil and paper or a slide rule, 22 divided by 7 is a good enough value of π for all practical domestic purposes. (To divide by this, multiply by the 7 and divide by the 22.) If you want to know more (there is a lot more!) key 'pi' into Google.