Lacquers are materials that are applied dissolved in a solvent, which later evaporates to leave the main bodying ingredient. Shellac is considered a lacquer, for example. Nitrocellulose and acryic lacquers are the other generic varieties. With the exception of water-borne lacquers like KTM-9, there is very little post-application curing going on.
In contrast, varnishes are generally composed of either natural or synthetic resins dissolved in an oil medium, like linseed or tung oil. The oils must cure by crosslinking in order for the finish to harden and that crosslinking entraps the dissolved resins, which endow the finish with whatever flexibility, hardness, and color the formulator is targetting.
Thanks. I got a catalog in the mail for violin makers that has the resins. It also has the oil mediums, I guess. I know what laquer is composed of, but I've never found a clear description of varnish. Can you give me a quick description of crosslinking? Does it require a catalyst?
Crosslinking is just a chemical reaction between two molecules that permanently links them together. The oils in the varnishes are composed of very large molecules that have more than one reactive site on them so that one molecule can react with another molecule, that can react with another molecule ad nauseum. At some point a very tight mesh is created that is no longer soluble.
The reactions don't occur spontaneously at a significant rater. Otherwise, varnish would have a real short shelf life. For varnishes containing linseed and tung oil, the crosslinking is best catalyzed by sunlight, I believe because of the high content of UV-B.
Modern synthetic varnishes can be of two types. The first is 99+% one crosslinkable ingredient, to which one adds a catalyst. The other type of varnish is formulated as two components that start reacting as soon as their mixed and have a pretty short pot life.
I wonder if oxygen is involved in the chemical process of the varnishes?
I was told long ago to store raw linseed oil in the window
so that the sunlight could do some good things to it.
I was also supposed to sink some marbles in the bottle,
to always leave a minimum of air in it.
The bottle I used is that brown pharmacy type - I believe it
does keep some of the UV-light out, but I have had it in the
window for more than ten years now, with very little taken out from it.
Do you think there is a rarity in that bottle - or was it a myth only?
For starters, glass filters out most UV light except for the longest wavelengths, which really don't have the energy required for the associated photochemistry. Doesn't hurt that it's a brown bottle, either. Not sure about storing it on the window in the sunlight. Oxygen does participate in the crosslinking reactions but it doesn't take much. Remember the warning about storing rags exposed to varnishes and turpentine in closed containers? Just a little oxygen can initiate a sufficiently exothermic reaction that the stuff will catch on fire. That's why it's recommended to store/dispose of the rags in containers of water. The water has a high heat capacity and keeps the reaction from getting out of hand. Works for nuclear reactors too. Well, most of the time.
If you had stored your varnish in a polyethylene bottle, you might have seen more change to the stuff.
Also some varnishes have heavy metal oxides added to them to help in the hardening/drying process. You might be able to find some very detailed information on the Southern Calif. Violin Builders forum. It's been a long time since I have been on there but you probably know that violin folks attribute magical properties to varnishes and various gums,resins and coloring materials that they use to make the stuff. Making your own varnish is an excellent way to meet the guys from the local fire station so be careful if that is your intention.
Yes - I got the bottle-in-the-window myth from a violin builder 30 years ago.
Unfortunately the oil has been exposed to a very small amount of oxygen,
despite the marbles.
It seems more transparent and colorless than fresh oil,
and there is a small change in viscosity,
so - probably not the collectible I hoped for.... :-)
Here`s some information I wish was around years ago.In the STRAD magazine FEB 2009 p58. How to apply varnish.Using solvent free varnish.The method uses prosthetic foam instead of a brush and you get about 20 minutes to work with it before it gets too tacky.All that is in the Trade Secrets series
The gentleman in the article has won gold medals for his violins but you can use the same varnish if you want.
The varnish is made by MAGISTER VARNISH PRODUCTS and you can download enough information to choke a horse.Never seen so much useful free info in my life.All made in Holland I believe.
Back copies of STRAD mag easy to get.