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When making an Ambrotype, you can paint the back black or seal the image by pouring on warm varnish (110 degrees F)
Sanderac Plate Varnish220 ml Grain Alcohol 190 proof (substitute with denatured alcohol if needed)38 grams Gum Sanderac22 ml Lavender Oil NF2 ml Distilled water (x 2 if using denatured alcohol)
Add the sanderac to the alcohol in a mason jar (or similar). Cap securely and periodically shake vigorously on and off over the course of a few days until the gum sanderac has all dissolved (leaving only the bits of bark, bugs and dirt). Add Lavender oil, water and mix.
Let the capped jar sit for several days to settle, then decant the cleared varnish using a lab pipette, eye dropper or a small "baster" (that thing used for the Christmas Turkey) and filter it into a separately stoppered bottle.
Using an oil lamp, heat up your plate carefully and evenly, making sure you can still touch the underside. Pour your varnish into the middle (or into a corner, depending on your technique) and flow it evenly over the plate. Try not to let it run back onto itself. Let any excess flow off from a corner into a separate bottle - do not pour it back into the main supply!
Heat the plate evenly over the lamp's fire again, again being careful not to overheat it or to set it alight (just blow it out if it does) - this will cause bubble in your varnish - once it gets tacky, set the plate away in a dust-free environment to dry completely. If you pour your plate unto a non-heated plate, the varnish will be cloudy or milky in appearance. Varnish can be applied a second time if wanted to build up the gloss.
Do NOT shake or wildly stir your varnish after filtering as it will cause bubbles to become trapped in the liquid.
Alternatively (ONLY when using clear glass):
Place your fully dried Ambrotype onto some old newspapers, with the silver image side up and spray with black car paint. Leave to dry. The image is not fully sealed and can be viewed through the clear glass backing. Obviously not recommended for posterity / archival images or ones that require them to be historically correct.