Home
About the Book
Book Outline
Contents
Contributors
Peer Review and Editorial Board
Design, Production, Sample Chapters
Technical Schemas and Diagrams
Original Artwork
Publication Schedule
Who We Are
How Sales Will Support the IPCI
About Pernambuco
Links
News
Order
Pre-Publication Offer
Order by 30 November 2009 and receive a $200 discount.
Home
» Replacing a Pearl Eye in the Frog: An Improved Method Using Hydrochloric Acid
Replacing a Pearl Eye in the Frog: An Improved Method Using Hydrochloric Acid
Embedded Scribd iPaper - Requires Javascript and Flash Player
Enable JavaScript in your browser to view this document as it was initially formatted.
Replacing a Pearl Eye in the Frog: An Improved Method Using Hydrochloric Acid David Hawthorne
This technique uses hydrochloric acid to carefully dissolve the excess pearl in a newly installed eye until it is flush with the ebony surface of the frog. The great advantage of this finishing treatment is that the contour of the ebony and the irreproducible patina of age are kept safe from filing or abrasives, while the pearl itself is shaped very controllably. Some restorers have reported bad results from dissolving excess pearl with other acids, such as nitric acid, which is often used in a concentrated form to stain the pernambuco stick of a bow. In a test I conducted on several pieces of ebony, including old finished frogs, nitric acid bleached the wood and left traces when it was dry, but hydrochloric acid did not. Moreover, the erosive action of hydrochloric acid is quite controllable, and since it is a component of human chemistry, it is arguably no more foreign to the frog than sweat. The patina on ebony, however, is very susceptible to alteration, and even the water in an acid solution or solvents may, in some instances, affect it. Nonetheless, I find this technique to be the most satisfactory approach to pearl eye replacement. It can also be used to replace the eye in the end of the button and to finish the replacement of a pearl slide. In the latter case, this should be done off the frog and with the sides protected with beeswax. Any restoration technique requires care and attention to avoid damage to existing work. The cardinal rule of instrument restoration is to safeguard what the original maker created and, whenever possible, to only make
changes that are removable or retreatable. Bearing in mind that the pearl pieces on a frog must be replaced from time to time, this technique follows that rule when it is faithfully executed as described below. A word of warning, however: this procedure requires precise filing next to surfaces that must not be damaged. If in doubt, practise on student-quality bows until you can carry out the procedure flawlessly. Most replacement methods are detectable to the trained eye. The least desirable method is to fit and glue the pearl eye, then file it flush with the ebony surface, as if making a new bow, and in the process, file away some of the wood. This method would only be suitable on the cheapest of bows, and even then, it is not as quick as the method I use, since time must be allowed to repolish the filed surface. The most typical replacement method consists in placing the flat eye in the concave side of the frog without matching the pearl’s surface to the contour of the ebony. Though the result of this method is aesthetically disappointing, its use is understandable from the standpoint of conservation, since the pearl eye is completely adjusted to size and fit before it is installed into the frog. The possibility of damage to the original frog is minimized, since no filing on or near the wood is necessary. A more refined method, and one making more demands on the restorer’s skill, consists of shaping a con-
Figure . Eye before repair. This pearl is corroded through from the player’s sweat.
cave eye to match the curve of the frog before gluing it into the frog. Great skill is needed to duplicate the exact contour of the frog onto the pearl, glue in the eye so that it lies flush with the surface and achieve a tight fit with no glue showing. I have seen some less-than-perfect instances where the eye had moved slightly during the gluing process, so that the pearl protrudes where it is misaligned. A common feature of the techniques just described is that the repairer often will make the eye somewhat undersized to fit and adjust it more easily, which results in a noticeable glue line and calls attention to the repair. Using the method I describe, the restorer can obtain a tight fit, a minimal glue line and a perfectly flush surface (or one where the eye is slightly dished in, as it is on most old bows) as well as a correct direction of iridescence – all without damaging the frog. An eye may need replacing because it is eroded through, cracked or missing entirely (figure ). The first step is to remove the old pearl and glue without damaging the pearl bed or the surrounding ebony. If the eye is mostly intact, begin by drilling a hole into the centre as deep as the pearl, without drilling into the frog. Pick out the larger fragments by carefully prying and lifting them with an awl. Eyes in old frogs were almost invariably glued with hide glue, which you can soften and remove by dabbing a little hot water onto the pearl bed with a small brush. If more recent work has been done, the eye may have been glued with CA glue. In this case, dissolve it with CA solvent. Polyvinyl acetate (PVA) glues, such as Titebond and Elmer’s, dissolve grudgingly with vinegar but are rarely used on fine instruments and bows. If the eye is surrounded by a silver ring, be careful not to saturate the area with water while removing the old glue from the pearl bed. Since the ring is likely also set with hide glue, the glue under it could easily swell and push the ring out, significantly complicating the repair. Using an awl with a square cross section, clean along the edge of the hole and down into the corner as you soak out the glue with the appropriate solvent and without damaging the face of the frog. The edge of the awl’s square
shaft allows a slight scraping action. Do not be tempted to use a chisel to clean out the corner. The trailing edge will surely nick the margin, marring the appearance of the finished work. Removing the old pearl and cleaning old glue from the bed can take anywhere from minutes to an hour. Do not rush this important step; it is essential that the pearl bed be completely clean before you fit the new eye. • Mother-of-pearl, in sections or cut from a slide or shell • A dowel a little smaller in diameter than the eye you are replacing and a few inches long • Regular clear CA glue • Black Max “toughened” CA glue (I recommend Loctite 380 instant adhesive, Black Max part no. ; see Notes on Materials.) • CA solvent • Plastic tape, such as Magic Tape • Almond oil • Files to shape the eye: A Grobet cut # fine crossing file, which has a lenticular cross section An F. Dick German cut # medium-fine half-round needle file A Grobet # flat file or a single-cut mill file • A x magnifying loupe • A flat wooden stick, such as a coffee stirrer, to spread the glue • Paper towels • Four-inch cotton swabs • A bottle of concentrated hydrochloric acid (The acid I use tests on a pH strip, indicating that it is close to the highest concentration level.) The dedicated restorer will have a selection of pearl in stock so that a good match can be made with the other pearl on the frog. I do not recommend precut pearl eyes in the case of a fine restoration, since they rarely fit the diameter of the pre-existing pearl bed and are not often selected for best appearance. The pearl should be about
Figure . Shells for eyes.
Figure . Eye ready to shape; checking for best reflection.
. to mm thick. In the case illustrated here, I am making the eye from baby abalone shell (figure ). Cut a square piece of pearl about mm larger than the diameter of the original eye (and bed). File it down to a perfect square, still oversized, then to a symmetrical octagon. To facilitate shaping and fitting the eye, glue it to the dowel with regular CA glue, with the side of the pearl that will ultimately show facing the dowel end. Before gluing, identify the direction of iridescence by rotating the piece until you see the brightest reflection (figure ). Look at the pearl in sunlight – a bench lamp’s directional light can be misleading – and mark on the dowel what point on the circumference will correspond to “twelve o’clock” when the eye is installed. The installed pearl should reflect best from the following views when the ferrule is on the left: • From the side of the frog, with the pearl slide facing down. • Rotated ˚, with the pearl slide facing you. • From the other side of the frog, with the pearl slide facing up. Almost all handmade bows observe this protocol, and so must the careful restorer. However, there are rare old exceptions or else hastily made examples. In such cases, repairers must use their own best judgement as to whether they “correct” the problem or assume that the maker had a different idea. Now, with the eye glued onto the end of the dowel, file it to accurately match the shape of the pearl bed. File off the eight corners of the octagon, the sixteen resulting corners then any bumps to form an arc. I find that achieving a perfect circle is simply a matter of filing away all the bumps and irregularities from the outline. No bumps: perfect circle! As you finish, bevel the edge of the pearl disc very slightly, making the diameter of the face larger than the diameter of the back. This will minimize the glue line around the finished eye. Try the eye in the pearl bed as you file it to shape. If you find that the pearl bed is slightly oval, you will want to adjust the shape of the pearl to correspond. When the eye almost fits, shiny spots on the edge of the pearl disc – burnishings caused by twisting the disc in the hole – will show the points of contact and help you adjust the shape. File those points to achieve a perfect fit, using the loupe to
David Hawthorne
Replacing a Pearl Eye in the Frog: An Improved Method Using Hydrochloric Acid
check the contact between the circumference of the pearl eye and the circumference of the pearl bed. The pearl should sit proud of the ebony surface by about . mm, and higher in the middle, because of the curvature of the frog. As you seat the pearl, bear in mind that you will lose some of the extra thickness when you finish the surface with the acid. Use the Black Max CA glue to glue in the eye. Black CA glue is not as brittle as regular CA or hide glue, so the eye is less susceptible to being dislodged by an accidental blow. Spread the glue in the pearl bed with the wooden stick and carefully twist the eye into the recess with the help of the dowel. Since the edge will be obscured by the liquid glue, use your fingernail to ascertain whether the eye is set in just right. You will have between seconds and minute before the glue takes hold (figure ). Cure several hours, or even better, overnight. Snap off the dowel then clean off all excess glue with CA solvent. The next step is to file down the pearl close to the ebony. Before proceeding, protect the frog’s side face with plastic tape, but first, rub a little almond oil on the ebony to avoid pulling up small splinters when you eventually remove the tape. Masking tape is unsuitable here; you need the smoothness of plastic tape. Be sure to cover the whole side of the frog, behind the eye and in front of it, including the ferrule and thumb extension, which can easily be damaged as you file the eye (figure ). Beginning with the fine crossing file and finishing with the flat side of the medium-fine half-round needle file, carefully smooth down the pearl to the limit set by the tape. Once this has been done, the pearl will stand proud of the surface of the ebony by exactly the thickness of the plastic tape, and the file will not have touched the ebony (figure ). Be very careful not to tear the tape and mar the ebony while you file. One mistake could negate the advantage of this whole process. After each file stroke, blow or wipe away the pearl dust and examine the result. If the tape begins to tear, stop immediately, remove the damaged
tape and replace it. After filing the eye, carefully remove the tape. At this point, we are ready for the acid treatment. Take great care when using acid: wear suitable eye protection, rubber gloves and perhaps a rubber apron, and keep some baking soda and a bucket of water ready for accidents and spills. Be careful not to breathe in the fumes, and make sure you have adequate ventilation. Dip a long cotton swab in the bottle of concentrated hydrochloric acid and rub it firmly over the pearl eye, moving back and forth lengthwise in relation to the frog (figure ). The acid will foam as it reacts with the pearl (which contains calcium, a base). Check the progression of the erosion frequently by blotting the acid with a paper towel and feeling the pearl with your fingertip, with or without a glove. My fingers have never been harmed by the acid, which is neutralized by the calcium in the shell. But be cautious. Your skin may not be as thick as mine. When the pearl wears away, a protruding rim of glue will remain. Dissolve it with CA solvent. Repeat the entire process as needed with the aciddipped swab, rubbing away some shell and cleaning up the rim of CA glue that protrudes around the pearl eye. Only one swab is usually necessary for the entire job. As you swab, you will be able to control the erosion of the pearl; the action of the acid slows down as the pearl neutralizes it. With judicious swab work, you will be able to match the contour of the frog quite precisely. Stop applying acid when the pearl is flush with the ebony, or continue until it is slightly dished in, as is typical on old bows. Try to match the other pearl on the frog, unless they must be replaced also. If the pearl is flamed, it will erode with a slight washboarding, as it does in the player’s hand. After the acid treatment, the finished surface of the pearl is a bit shiny and should need no other polishing. Rub a bit of almond oil and some wax on the ebony to shine it up and mitigate the drying effect of the acid and the CA solvent. For an even more natural look, rub rosin dust into the eye, letting it build up in the margins. If well
Figure . Eye glued in before dowel is removed.
Figure . Protecting side of the frog with tape.
Figure . Dissolving excess pearl with hydrochloric acid.
Figure . Tape after filing.
David Hawthorne
Replacing a Pearl Eye in the Frog: An Improved Method Using Hydrochloric Acid
matched, the pearl eye will be quite unobtrusive or even undetectable as a repair (figure ). ***
Notes on Materials and Tools Figure . The finished eye. Loctite instant adhesive, Black Max part no. , is not brittle like regular CA glue when it dries. It is less shiny when polished and matches the appearance of ebony better if any glue line is left. It also has a good shelf life. Four-inch cotton swabs can be bought at a good drug or surgical supply store. Get the ones with wooden sticks. Notes Hydrochloric acid, however, is not a component of sweat. The main organic constituent of sweat is lactic acid, and this is what eats away at pearl in the player’s hand. But a strong pH is needed to have this effect. Sweat is % water and electrolytes, which are mainly composed of sodium and chloride ions (giving sweat its slightly salty taste) and, to a lesser degree potassium, calcium and magnesium ions. The pH of sweat varies between . and ., in close correlation with the amount of lactic acid excreted. See Paul Siefried, “Replacing the Silver Ring of a Parisian Eye and Making Double Pearl Eyes,” xxx–xxx. For more information on diffraction, see Padraig o Dubhlaoidh, “Care and Conservation of Mother-of-pearl,” xxx–xxx. You could also use hide glue. The hot water in it will sometimes slightly swell the ebony around the eye and close very small gaps. However, though I have used hide glue for new eyes, I have never used it for this repair. The black CA glue will not give way, as long as it is properly cured.
David Hawthorne
This document is © 2009 by root - all rights reserved.
