In this varnish, you have the option to use nitric acid to color the resin. If you decide not to use the acid, skip step 2 in the Preparation. The varnish will be less colored and great for the finishing coats.

Equipment

1. Hot plate preferably thermostatically controlled, with diffuser screen.
   
2. Thermometer. (maximum of 350° C will suffice). A candy thermometer or a cheap Radio Shack multimeter with a thermometric probe.
   
3. An enameled or stainless steel pot.
   
4. Lab spoon or glass rod for stirring.
   
5. Scales.
   
6. Coffee filters.
   
7. Fire extinguisher. (ready at all times)
   
8. A pipette or a dropper.
   
   

Ingredients preparation

Liquid wood ash

1. Burn preferably hard woods, a mixture of spruce and maple shavings is acceptable.
   
2. Put an amount of ash that is equal of that of resin {200gr} in a coffee filter.
   
3. Pour an equal amount of distilled water {200gr} in the filter and wait until all the water gets filtered.
   
   

Linseed oil thickening

Cook 170gr of linseed oil for 1.5 hours - until it gets thicker and considerably darker.

Ingredients

1. 120gr Damar, 80 gr Calophony.
   
2. 65% nitric acid.
   
3. Linseed oil or Walnut oil Cold pressed.
   
4. 200gr Liquid wood ash.
   
5. 1/2 teaspoon - Slaked Lime.
   
6. Spirits of Gum Turpentine.
   
   

Preparation

1. Put 120gr of Damar and 80 gr of Calophony in your pot. Bring to 100C.
   
2. Bring to cca 170C and start adding 65% nitric acid. A pipette or an eye dropper are great for this. 50gr should be enough. Add a little, let bubble, stir, and when the reaction subsides, add more. {you can skip this step to obtain clear varnish}
   
3. Add 200gr of Ash water + 1/2 spoon of slaked lime, keep stirring.
   
4. Boil off all water.
   
5. Bring to 280C and boil for 10 minutes.
   
6. Thicken 170gr of linseed oil.
   
7. Warm up resin to 270C, add thickened oil.
   
8. Cook at 300C for approximately 30mins or more, until a successful string test, see bellow.
   
9. Remove from heat, warm up 100gr of turps to 100C.
   
10. Add stirring to 140C varnish.
    
    

Advantages

Gives a deep yelow/red/brown transparent color. Stable colors.

Disadvantages

Some people say the nitric part may negatively affect the varnish in time. If that matters to you, just don't add any.

String test

The string test gives you an idea about how well the resin gets bonded with the oil. A failing string test means insufficient bonding.

1. Place a drop of hot varnish into a glass of water, let cool for 5-10 seconds, pick up and pinch between your thumb and index fingers. As you pull your fingers apart a string should form.
   
2. Failing that, continue cooking until varnish strings. The longer strings it gives you, the better. 5 cm would be great.