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xraymymind

Varnish making... why add Lime?

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Hey all.

this is a question to all of you out there who add Calcium Oxide, or Hydroxide to your varnishes on cooking. I am wondering what the actual reason for doing this is: what qualities does the Lime add to the varnish?

i have made a great varnish cooking lime in with Greek pitch, but was never actually sure what benefits/drawbacks adding the Lime actually had. I have also heard of people cooking it in with their Linseed oil prior to amalgamating with the GP. Why add the lime to one and not the other, etc?

Let the can of worms be opened!

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10 hours ago, xraymymind said:

Hey all.

this is a question to all of you out there who add Calcium Oxide, or Hydroxide to your varnishes on cooking. I am wondering what the actual reason for doing this is: what qualities does the Lime add to the varnish?

i have made a great varnish cooking lime in with Greek pitch, but was never actually sure what benefits/drawbacks adding the Lime actually had. I have also heard of people cooking it in with their Linseed oil prior to amalgamating with the GP. Why add the lime to one and not the other, etc?

Let the can of worms be opened!

Both limed rosin and limed linseed oil are old and traditional (cheap) components of varnishes in the first half of the 20th century.   The calcium rosinate is less acidic than plain rosin and is also tougher.  

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15 hours ago, xraymymind said:

Hey all.

this is a question to all of you out there who add Calcium Oxide, or Hydroxide to your varnishes on cooking. I am wondering what the actual reason for doing this is: what qualities does the Lime add to the varnish?

i have made a great varnish cooking lime in with Greek pitch, but was never actually sure what benefits/drawbacks adding the Lime actually had. I have also heard of people cooking it in with their Linseed oil prior to amalgamating with the GP. Why add the lime to one and not the other, etc?

From what I've observed, this is mostly done with rosin varnishes to reduce the high acidity of the rosin. In my own experiments, a rosin-linseed varnish seemed to degrade pretty quickly if high acidity remained.

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43 minutes ago, David Burgess said:

From what I've observed, this is mostly done with rosin varnishes to reduce the high acidity of the rosin. In my own experiments, a rosin-linseed varnish seemed to degrade pretty quickly if high acidity remained.

I have wondered how resins and oils can be acidic, as acidity is defined by H+ ions in aqueous solution. I suppose it's because wood, resins, oils and even air always contain a few % moisture to allow acidic behaviour.

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Oils and resins are really fatty acids, so adding lime leads to saponification. Saponified oils and resons have drastically different properties than the raw fatty acids, although not as much as simpler fats like e.g. palm oil (look at your soap, it will most likey contain sodium palmitate, which is the reaction product of lye and palm oil). Calcium soaps tend to be hard and practically insoluble, which is just what we need for varnish.

Addendum: Aqueous solution is not needed to call an organic acid an acid, it is enough that the formula contains one or more COOH groups.

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21 hours ago, DoorMouse said:

In my experience liming makes a tough varnish that wears very little.  Not ideal for antiquing.

My varnish is heavily limed and responds perfectly to alcohol. That is what I use for wear imitation.

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1 minute ago, Michael Szyper said:

My varnish is heavily limed and responds perfectly to alcohol. That is what I use for wear imitation.

I meant to say that it doesn't have the fragility you can achieve with an unlimed varnish.  It doesn't chip off easily with a fingernail. 

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1 hour ago, Michael_Molnar said:

FWIW, @Roger Hargrave does not add lime.

 

Varnish discussion aside, I’ve read “Making a double bass” PDF from Roger’s site several times. It should be required reading. Thanks for linking the thread, which I only read as it was being written. I’m sure there are many other gold nuggets in buried in the 32 pages. 

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Adding lime to colophony based varnishes cannot be considered as a factor on it's own.  When (and if) to add lime...and how much...is also a function of the way the oil and resin were prepared and the end goal of the varnish maker.

As with most of violin making it is both simple and  complex.

on we go,

Joe

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The reaction of rosin and linseed oil is chemically a cross-esterification. Lineolic acid gets attached to abietic acid. In solution such reactions are base catalysed. I assume that something like this is happening in hot rosin, especially, as the lime is added in water. Lime light just catalyse the ester cleavage of the abietic acid. Industrially anothet catalyst is used which I need to look up. This is just an assumption, as I know little about such chemistries.

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If you want to neutralize acidity without hardening the resin too much you could also use potassium or sodium hydroxide and/or potash.  I like to add 1/3 of the acid number equivalent, otherwise things get too hygroscopic. (By liming the rosin you actually are making soaps).

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