Hi Woodreaux, on reflection I suspect the reason for the oil of spruce in the Sheraton recipe may well simply be for its scent. Without that the recipe is similar to all others.
I've done a lot of digging into this subject over the last week. It's useful, IMHO, to reflect on the art world and how and why certain of these components are used. Here are a series of notes I've taken thus far.
* Linseed oil is a 'drying oil’. "Linseed oil is a triglyceride, like other fats. Linseed oil is distinctive for its unusually large amount of α-linolenic acid, which oxidises in air.” "Having a high content of di- and tri-unsaturated esters, linseed oil is susceptible to polymerization reactions upon exposure to oxygen in air. This polymerization, which is called autoxidation, results in the rigidification of the material.” Wikipedia
* One issue with linseed oil is its tendency to yellow over long periods of time. As a result, "Linseed oil use has declined over the past several decades with increased availability of synthetic alkyd resins—which function similarly, but resist yellowing.”
* A ‘varnish’ is a combination of a solvent (eg turpentine) and a resin Resin (eg Venice turpentine)
* Resin is added as a softener or plasticiser, to counteract hardness or brittleness in a varnish compound. (A small amount of beeswax can be used in the same way.) https://www.jacksonsart.com/blog/2022/03/10/larch-venice-turpentine-a-resin-not-a-solvent/
* When a varnish is combined with a drying oil, it’s called a ‘medium'. A medium is a liquid that is added to oil paint in order to change the performance of the paint in some way.
* Many companies now produce a range of artist mediums than vary in their drying time and other properties (eg viscosity)
* Driers help speed up the ‘drying’ process of a medium. Wikipedia
* Terebine driers, in the UK, is a blend of metallic driers formulated to speed up the oxidative drying of solvent-based oil and alkyd based paints, stains and varnishes. The US equivalent would appear to be ‘Japan drier’, another generic term, which is largely naphtha plus cobalt and other metallic components. The name comes from the 17th century European practice of "Japanning," a process designed to imitate Asian lacquerwork. https://www.facebook.com/UtrechtArt...ht-japan-drier-is-a-/10154485032236704/#
* "With an alkyd resin base and the tendency to alter the appearance of oil colors over time, Japan Drier is not recommended for fine art use. When left to dry to a high tack, Japan Drier can, however, be used for gold and silver leafing.” "Driers made from lead, manganese and other proprietorial mixtures that do not wholly contain cobalt driers are to be avoided as they promote darkening and loss of flexibility."
* Cobalt drier is strongly recommended as a drier for fine art oil painting. "Cobalt Drier is the only siccative that has been scientifically proven to be the least harmful to use for fine art painting. Cobalt Drier speeds drying of oil colors and oil painting mediums. Because it is extremely strong, Cobalt Drier should be used sparingly by applying it in drops. It should not be mixed directly into oil paints, but first mixed into an oil painting medium, which can then be added to the paint.”
* A cobalt drier accelerates the drying time of linseed oil from 2-3 days to 1. See table of drying rates: http://langridgecolours.com/cobalt-...embrittlement%20of%20the%20paint%20film.
I'm not surprised that Dig switched to adding 'Terebine' driers. Without a drier it takes linseed oil 2-3 days to 'dry'. I've made up a batch according to his original recipe (ie with the spirit of turpentine acting as a solvent alone) and will be adding a cobalt drier in the coming days to speed things up.
Hopefully these notes are useful to some!
EDIT: this remark in the Langridge page is informative: "Cobalt Driers cannot speed the drying of artists’ varnishes as they contain no oil. Artists’ varnishes dry by evaporation of the solvent." So if I look at Dig's original recipe, we have, I believe, a varnish - the larch resin (Venice turpentine) and carnauba wax dissolved by a solvent (spirit of turpentine) - mixed with a drying oil (linseed oil) to create a 'medium'. It has no drier accelerating the 'drying' of the oil. The solvent will dry by evaporation, but the oil drying is by polymerization (with exposure to oxygen). A solvent won't speed polymerization (although in oil painting it does thin out the medium, makes the paint leaner, and thus the paint will dry faster than a thicker, more viscous, non-dispersed, more oily paint film). But the amount of solvent needs to be significant for this to effect. ***
So our gunstock oil needs a drier to accelerate this polymerization process if we aren't to wait 2-3 days between coats (or we wish to have access to a tacky film (which is wiped off) to help speed the filling of grain. Hence the change to using Terebine drier. (I'd likely keep the solvent.) The cobalt drier mentioned above is very powerful. The recommended dilution is a mere 1% which is about a teaspoon addition for Dig's original formula.
*** If you look at the table in the Langridge link above you'll see that even a 'General Medium' of 1 part Linseed Oil + 1 part Gum Turpentine has an approximate drying time of 2 days. That's a 1:1 ratio. The ratio of spirits of turpentine to oil in the original formula is 1:8. ("Times listed are for paint films reaching a non-tack surface and not for complete through-film drying.")