Stradivari Varnish oil/resin ratio

[uguntde]

On 8/27/2020 at 10:06 PM, uncle duke said:

I'd put money on sospiri or uguntde having the theory.

My guess is an ingredient from the tube may have something to do with it.  Mine goes darker with some sheen loss - hope it stays where it's at.

edit - I use a quart glass jar for holding spirits of turpentine so that it can air out before use for varnish making - I notice the dried out residue on the inside of the jar has the same dullness/sheen that I think I notice on a few of my later violins - [recipe change].

It is a sheen that is glossier than a semi-gloss from the hardware store but not as luminescent as a violin by Mr. Ouvry that I have. 

Then there's always the possibility of linseed oil changing everything like Ugun explained above. 

Fluorescence?  I'm not familiar with what I should know about it- haven't reached my level of incompetence yet.

 

Fluorescence stands for a spontaneous emission of light after light exposure, whereby the emitted light has a longer wavelength than the original light source. I.e. you shine blue UV onto the varnish and it emits visible light, or you shine UV in freshly washed clothes and you get a white visible light.

Fluorescence can be quenched, in varnish, a well-known quencher is oxygen (this quenching effect is used in oxygen sensors). Once all oxygen has been used to cross-link the lipid chains it is chemically bound and the remaining conjugated double bonds will show fluorescence in absence of a quenching substance.

Importantly, it is still only those alpha-linolenic acid and linoleic acid molecules that maintain their conjugated double bonds that fluoresce. Those lipids that were cross-linked have lost their chromophores. I therefore assume that what we see shine in varnish is the free fatty acids.

An experiment to proof this would immerse lots of oxygen into the varnish - I tried this once and the varnish started to burn immediately. But maybe massive oxygen exposure during drying in the light box could be used as a test. The oxygen would speed up drying but quench the florescence effect. Otherwise an oxygen-free varnish would not be able to used oxygen for cross linking and instead dry through Diels-Alder type reactions. This may be slower but give a nicely shining varnish.

Another test would be to modify the oil/ rosin ratio. I assume it is mainly the oil that fluoresces. On the other hand abietic acids also has a conjugated double bond and may show fluorescence of its own, although almost certainly with another frequency shift.

 

 

[uguntde]

17 hours ago, Michael_Molnar said:

I’ll hazard a guess that you are seeing the solvent dominating the fluorescence at the start. As it evaporates or dissipates the roasted rosin is left to fluoresce. I’d need to do some simple tests to evaluate that hypothesis. 

Depends what the solvent is. Turpentine (alpha-pinene) won't give you a lot of fluorescence. I don't know what other solvents you use. Oils won't evaporate.Oil varnish drying is not an evaporation process as is the case for spirit varnish.

Try oxygen-free and oxygen-loaded linseed oil. Oxygen is known as a flueorescence quencher.

 

[sospiri]

2 hours ago, uguntde said:

Depends what the solvent is. Turpentine (alpha-pinene) won't give you a lot of fluorescence. I don't know what other solvents you use. Oils won't evaporate.Oil varnish drying is not an evaporation process as is the case for spirit varnish.

Try oxygen-free and oxygen-loaded linseed oil. Oxygen is known as a flueorescence quencher.

 

If the oil or varnish goes black as we see in some old instruments and paintings  is this because it is oxidizing without polymerizing?

[uguntde]

7 hours ago, sospiri said:

If the oil or varnish goes black as we see in some old instruments and paintings  is this because it is oxidizing without polymerizing?

Are you sure this is not dirt building up? There is no good reason why a black colour (massive absorption) should develop - at least I can't see this at the moment. In some of these Southgerman instruments the black colour is caused by the use of dichromate which is a massive oxidant. Theer the surface of the wood turns black. For which makers have you seen this blackening?

[JacksonMaberry]

I think iron oxide pigments also show black in UV but not sure 

[sospiri]

18 hours ago, uguntde said:

Are you sure this is not dirt building up? There is no good reason why a black colour (massive absorption) should develop - at least I can't see this at the moment. In some of these Southgerman instruments the black colour is caused by the use of dichromate which is a massive oxidant. Theer the surface of the wood turns black. For which makers have you seen this blackening?

I know that linseed oil can go a very dark brown even in very thin coats. I was wondering why?

I was thinking that maybe if it dries very slowly it is oxidizing and not polymerizing?