Prior to varnishing, you should set up the violin in the white and make sure it is playable, and it doesn't have any buzzes, which could require reopening of the instrument. Only if you are satisfied with the sound, should you proceed to prepare the surfaces for varnishing.

Preparing the wood surface
  1. Go over the whole surface of the violin and correct areas which you have overlooked before, or defects arising from manipulation. Be especially careful in the areas where grain sticks out, ie. overhang and make sure you make these places smooth without loosing the crispness of the ridge.
  2. Using a damp wash sponge or cloth, wet the whole violin with water to rise the grain. Let dry.
  3. Remove the grain everywhere using a fine scraper, knife as scraper and sandpaper. Be careful on the top plate where you shouldn't use any sandpaper, just scrapers to not loose the texture of spruce. For more see the previous chapter on finishing.
  4. Redefine the edges and chamfers if necessary.
  5. You may now selectively smooth out some of the features, ridges and chamfers if you feel the violin should look less edgy. Keep in mind though that the varnish itself will make the edges smoother.

Tanning overview

Now that your violin is ready we need to make the wood darker which will save us some coats of varnish and make the varnish look more attractive. At our disposal, we have a number of methods A-D, which we will discuss here in some detail.

Any of these procedures are best first tried out on scrapes of wood, so that you get an idea about the amounts and times involved.

A] Sodium nitrite + UV sun rays

You need:
  1. 4% Sodium nitrite, 50 ml. To prepare, weigh 50 gr of water and add 2 gr of Sodium nitrite.
  2. A sunny day or an UVB mercury lamp or an UV cabinet.

The process:
  1. Put a coat of 4% sodium nitrite on the violin. Make sure the coat is even and there are no drops left as these would make the wood darker. Don't forget to apply some under the fingerboard as well.

    Sodium nitrite accelerates what normally happens when you put the violin out in the sun. It helps oxidize the outer layers of the wood making it look darker.
  2. With the fresh /wet/ coat of sodium nitrite expose the violin to UV radiation. For wood tanning again exposure to direct sunlight or a strong UVB light is required.

    You have basically three options:

    a) Put the violin out in the sun. Get a length of firm string, tie the volute to one end and the other somewhere where the sun shines most of the day, ie a tree branch. The violin will rotate this way ensuring the proper distribution of UV. The only weak spot may be the underside where the button is, so put a sheet of aluminium foil right under with the distance of about 5 cm. The times depend on the strength of the sun. In summer a couple of hours should be sufficient.

    b) Use a UV mercury sun lamp, which emits strong UVB. With these precautions must be taken to avoid direct exposure of any part of your body, eyes especially.

    If you have the one bulb mercury lamp you will have to make sure, all parts of the violin get the same exposure. You can coat the whole violin with sodium nitrite at once though. Then expose different parts. The violin can be dry during exposure but higher humidity levels either in the violin /freshly coated/ or in the air help speed up the oxidative process of tanning.

    For a 250 Watt mercury UV lamp about 20-30 minutes from 40 cm distance are enough, assuming the violin is still damp from the solution.

    You will need to rotate the violin every 30 minutes to cover all angles. Don't forget to turn off the lamp before you do this, or wear UV protective glasses. It takes a whole day to tan a violin this way.

    c) UV cabinet. The black light UVA-B tubes used in varnish curing are usually much slower in tanning so you will need the white ones which don't block out most of the UVB.

    Tanning times for the whole instrument may vary from hours to days depending on the kind of tubes you are using. Watch for humidity, see the varnishing section.

Sodium nitrite precautions

Though commonly used as food additive (E250), in higher amounts it can be toxic and lethal to animals and humans. Human lethal dosage is 71 mg/kg, meaning a 65 kg person would likely have to consume at least 4.615 g to result in death.

UV exposure precautions

Wood tanning by exposure to UV radiation is inherently a destructive process so it should be practiced with caution. The desirable darkening of the wood is caused by the uppermost layer of the wood getting oxidized and degraded. The UV light produces ozone, which reacts with the nitrogen gas present in the air and forms nitrous oxygen. The nitrous oxygen then "burns" /oxidizes/ the wood leaving it slightly acid.

UVA can cause irritation of the eyes, prolonged overexposure may lead to some damage.
UVB can cause irreparable damage to the retina (welders eye) overexposure may cause sunburn and skin cancer.
UVC carries seriously increased risk of all of the above and it really should not be used in UV cabinets.

Also any type of UV light will produce some ozone and nitrous oxygen, the UVB/C lamps produce the greatest amount of these. They both are classified as poisons {depending on concentration} so precautions should be taken so that the room is sufficiently ventilated.

B] UV sun rays

If you want to avoid using sodium nitrite or are unable to obtain it, you can still expose the violin in the white to sunlight to darken naturally. Without any accelerator though, the violin takes a considerable time to darken. Exposed to direct sunlight, the stresses in the wood would be immense, humidity levels dropping which would probably result in numerous cracks. So instead of baking the violin in the sun, hang it somewhere with only indirect sunlight and wait... a couple of months. This is still possibly the least structurally invasive method, if done properly and if you have the time.

C] Ammonia fuming

In case you can't or don't want to use sodium nitrite as tanning accelerator, you can use ammonia fuming instead. This method offers uniform colors, color stability, brings out the annual rings in spruce. The tan can turn a bit greenish. If it is a problem, higher temperatures supposedly get rid of that. Maple can get less affected by ammonia, applying tea prior to ammonia on those parts can help. The long-term effects of ammonia on resonant wood are unknown, some makers say it even improves the wood while others fear it may adversely affect the instrument's structure.

You need:

1. An airtight box large enough to accommodate a violin + a cup of ammonia. The violin should preferably be hanged.
2. 26% ammonia /laundry grade/

The process:

1. Put the violin in the box together with a cup o ammonia.
2. Let it stand for a couple of days, up to five, or until you are satisfied with the color.

D] Staining

Staining can be used as an add on method to complement the tanning methods mentioned above. You can stain the bare wood with commercially available stains or use home made stains such as the black tea stain.

Category: Varnishing
Comment by
2016-09-10 07:55:31
I wonder if Antonio mixed borax with the water ( as a preservative ) on the entire violin to raise the grain before the final scraping / varnish . I've always wondered this since, after watching a documentary on Strad violins, they found traces of the chemical in the wood. Just a random thought.