Transparent Iron Oxide Dispersions

For coloring violins, many makers use madder root. David Rubio (deceased) described how he prepared madder root.
Madder is an organic compound. My understanding is that organic pigments and dyes are not as stable as some inorganic substances like iron oxides.  The difference between a pigment and a dye is, in simple terms, a matter of particle size. Paint contains large particles that scatter light, thus rendering the paint opaque. Normally violin makers do not seek opacity, but rather transparency.

Dyes can be added to varnish. But I am not currently doing this. I am concerned about longevity of the color, or lightfastness.

Madder root can be processed so that the particles are very small and a transparent dye-like substance is formed. I first read about transparent iron oxides in 2000. These particles can be made smaller than half of the wavelength of light. When this happens, the particles do not cause scattering of light. Thus cloudiness or opacity is eliminated. Unfortunately, when the particles are that small, they tend to clump together to make larger ones. Remember they contain iron, a magnetic element. This clumping is called "agglomeration."

The work-around is to prepare something called a "dispersion."  The manufacturer mixes the pigment with special chemicals and obtains a paste. I have many samples of these. BASF sells Sicoflush, which is one brand. Johns-Matthey in the UK used to make them, but I think this company has a different name now. Many are made in China.

Now, even having a dispersion is still not enough. One cannot simply mix the dispersion with varnish. One needs to do things like add the pigment to a solvent, and mix. Then, add this to more pigment and mix.

From TRAFe Dispersions:
"Application Notes --  If particles form when adding transparent iron oxide dispersions into the binder, this is caused by the fast reaction between the dispersion and viscous binder. Take the following two-step approach to avoid the problem:

1. Take required quantity of transparent iron oxide dispersion, and add solvent or water to lower its viscosity. The quantity of the solvent or water needed should be such that the pigment loading in the dispersion be lowered within 10%. Then take the appropriate amount of binder, which should be 5%-10% of the dispersion quantity, and slowly add it into the above dispersion while stirring in order to obtain low concentrated solution.
2. Stirring slowly, add the above low-viscous solution into the remaining binder."

I have not totally been satisfied with this process yet. I test tranparency by holding a glass slide up to light in a darkened room. From an oblique angle, one can see cloudiness if it is present. But I have a work-around to the mixing problem. Dispersions exist both for solvent based varnish and for water based varnish.

My solution is this: After the ground coat, and after making sure the surface is very level and smooth, I spread a thin coat of linseed oil. Then immediately apply the transparent iron oxide with my fingers, and then try to wipe away most of it. David Rubio shows this process with Madder Root. This process is called creating a glaze. The violin is placed in sunlight, or UV until the thin linseed oil film dries before the next coat is applied.

I found that I can easily control the color this way. It does take several coats, but I prefer this method.  Typically golden yellow is in the ground and the pigment coats are brown and then red, or some combination. Then, after the color coats, a clear coat is applied.

Dec. 28, 2009 Update:
Reports from violin makers are very positive about the use of LSB201 Red dispersion in oil varnish. Violin maker David Burgess got the ball rolling by showing a sample at the VSA convention this year. It is not completely transparent, but certainly more than passable. The BASF Sicoflush L version is probably nearly the same, but this product is not available in the USA. Therefore the LSB201 Red is being imported in small amounts from  Shangyu Chemical Industry Corp. in China. After further testing, efforts will be made to make this product available to violin makers.

Jan. 28, 2010 Update:
Problem solved (fingers crossed)!  The nice people at BASF responded.  I have it from a reliable source that Sicotrans L Red 2817 can be mixed with Laropal A 81 and oil to produce Sicoflush L Red 2817. Hopefully this will solve the problem. Violinmakers can store the powdered Sicotrans and the solid Laropal A81 and mix (mull) as needed. I am currently trying to obtain the Sicotrans L and the Laropal A81 to try out this method.

Jan. 30, 2010
Here is a patent that is actually readable about grinding iron oxides. And here an old patent for preparing the oxides. Also, I did a quick mulling of Inxel Red with linseed oil. It looks like this does have good possibilities. But someone would have to work out the details of the mulling process. Here is a paper about the problems associated with milling nano-particles. A company that mills dispersions is Dynamic Dispersions, located in Louisville, KY, USA. A company that has some good information and sells bead mill beads is Quackenbush. In particular, a good page on grinding issues. Glenmills grinding media pricelist. A sieve mfg. is here.

To make Sicoflush L from Sicotrans L, one ingredient needed is DISPERBYK-106. Another is Aerosil 200. Still another is MPA, or  1-Methoxy-2-propanol. Grinding plan:
Grinding resin: Laropal A81, 60%Xylene/MPA 1:1   ---- 25
Solvent: Xylene/MPA 1:1                                         ---- 37.5
Dispersing additive: Disperbyk 106                           ----  7
Anti settling agent: Aerosil 200                                  ----  0.5
Pigment: Sicotrans Red L 2817                                ----  30

Feb. 5, 2010
Patent applied for notice:  (Just kidding.) The process of applying by finger powdered Sicotrans L to a wood surface that is already wetted with a rubbing type varnish like ACE Spar varnish, has been discovered. Patent claims that the de-agglomeration process is accomplished by the friction of hand rubbing. Therefore no preparation of the Sicotrans L in a mill or mixer is needed. In particular, if maple is first prepared with a yellow ground, and smoothed with #400 abrasive paper, and the the above application of Sicotrans L red is accomplished, the result shows adequate de-agglomeration of the iron oxide to ensure transparency.
Inxel red, when rubbed in on oil seems to work ok too. I am in the process of evaluating the transparency.

Feb. 13, 2010
Trionix pigments are supplied by E. M. Sullivan Associates, Inc. (www.emsullivan.com). Trionix are made by Novant Chemicals. A good (old) webpage explains transparent oxide pigments here. 

Feb. 20, 2010
"The prior art preparation of iron oxide pigments is well known. Reference is made to various topics concerning iron oxide such as 'Pigments (Inorganic),' pages 816-818, vol. 17, Encyclopedia of Chemical Technology, by Kirk-Othmer, 3rd edition, published by John Wiley & Sons.
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