John Harte

I am curious about your statement regarding Echard. In what respect was his work not considered entirely reliable?

Matthew, a couple of past Maestronet threads that may be worth taking a look at are:

I have occasionally got this result. Sometimes it seems due to the cooked resin and oil not having combined as well as you would want. If this is the case, a small amount of heat under an IR lamp may be enough to get rid of the cloudy look. However it is generally better to try heating the varnish back up to a good cooking temperature, giving it more time at that temperature. If you do try this and have already added turpentine, be very careful and patient as you approach the boiling point of the turpentine. Without proper combining of the various components, these may separate out in the jar over time, at least in my experience. It could be that the colophony you are using has low resin acid content. Some years ago I had several attempts at making varnishes using Norway spruce callus resin. The end result was similar to what you have produced. I subsequently discovered that Norway spruce callus resin has particularly low resin acid content and have since wondered whether this was behind my lack of success. If my best guess regarding this is correct, in your case low resin acid content could be the result of some sort of treatment involved in producing the original colophony or you cooking the colophony at a too high temperature or the original resin merely having low resin acid content to start with. Is there any turpentine involved in your latest batch? If so, I would be looking at that as well. I also would think about the oil you are using. In your case, what exactly is boiled linseed oil, purified? Finding a good reliable resin source, oil and its processing that works and a good source of turpentine are not trivial. And then there is the establishing of a cooking process that reliably works, part of which comes from getting to know how each of these components behaves individually and together over ranges of temperature and timings. While this latest batch may seem to be a failure, it potentially is a huge opportunity to learn far more than an apparent successful result might provide. Varnishes that initially seem okay can over time turn out to be anything but...

Wow, that's impressive!! I take it that you are not hand sieving. Hand sieving a 20 micron sieve is enough to drive me to distraction.

Mike, my question was specific to burnt umber, not burnt sienna. According to the literature umbers contain between 5 to 20% manganese oxides and hydroxides. The manganese in burnt umber can apparently act as a surface drier. I am not thinking of burnt umber being cooked into varnish as a colouring agent but rather into oil as a surface drier. The literature suggests that this is possible. I'm not so sure about assuming modern pigments will all be of a suitably small particle size. In my experience simply mixing into varnish with a spatula can sometimes result in a quite granular and less transparent result with pigment particles in the dried varnish able to be seen with the naked eye. Mulling into oil using a muller and glass plate and then adding this into varnish generally produces a better result. Even better is using something like a 20 micron scientific sieve to ensure fine enough grinding. Not much of most commercially available pigment will pass through such a sieve. (Maybe or maybe not of relevance, most particles within Stradivari varnishes seem somewhat smaller than 20 microns.)

Mike, would manganese present within burnt umber be able to react with linseed oil? My understanding is that it can; i.e., when burnt umber is cooked into linseed oil. Have you tried grinding red lake pigments (e.g., madder or cochineal lake pigments) into varnish? What pigments have you tried and what method(s) have you used?

Yes I did and should have checked the text associated with this image in the Toscano book. The researchers state that these are iron based pigments, probably red ochre. They also list the trace elements present in the EDX spectrum of the particles. There is also further comment in the main text covering the analysis of this varnish.

Would you also think that burnt umber would be very unreactive? I am guessing so but would like to hear more. Hematite is sometimes mentioned as a presence in Strad varnishes. (See last sentence on second page of https://www.academia.edu/13260946/The_Nature_of_the_Extraordinary_Finish_of_Stradivari_s_Instruments ) Apparently heating natural siennas and umbers at high temperature causes the present hydrated form of iron oxide to dehydrate and transform into anhydrous hematite.

For some years I have been cooking small amounts of burnt umber into some of my varnish oil but, as you mention, not much colour is generated. I have not tried cooking burnt sienna into oil. I have only tried it ground into oil and then added into varnish but have not been overly taken with the outcome. As you say, always more questions than answers...

Thank you for these observations. You may be correct regarding the nature of the colouration of the 1690 Toscano varnish. Based on what I have read and seen or not seen in published microsample images, it seems that there is a lack of obvious pigment presence compared to later dated examples. Interestingly iron, and to a lesser extent manganese, are a noted presence in the 1690 Toscano varnish, both in best conserved and worn out areas. (See Figure 5 on p.66 in this book: https://edizioni.scrollavezza-zanre.com/en/tuscan-stradivari-violin-volume ) Maybe along the lines of what you suggest, might burnt sienna or burnt umber have been cooked into the oil as either a colourant or drier or both?

Dave, I have my bat ready to go but I don't think that I will be hitting the ball out of the park for you. Still, here goes.... Coloured particles (levigated as you describe) can indeed be used in a calcium caseinate sealer and can produce the effects that you describe. However, based my experience, this will struggle to produce the appearance (both on a macro and micro level) of, for example, a Strad ground. I also suspect that your approach will not fluoresce under UV in the same manner that Strad grounds generally seem to, not that we generally appreciate the qualities of particular varnishes based on how they fluoresce under UV... Your approach should obviously be governed by what you are trying to achieve. However I am not sure what exactly you are trying to achieve. If your goal is to reproduce the sort of features seen in some of the photos of old Cremonese instruments that have been posted here on Maestronet, then there are other approaches that might get you closer. I also don't quite understand what you mention in the last paragraph above your first photo regarding flame fade and oil and varnish producing the opposite effect. I also don't understand what you mention beneath your second photo regarding red pores crossing over the grain below like stitches. Would it help if I posted a video of an oil varnish ground to illustrate how variation in incident light angle changes flaming from light to dark and vice versa? Maybe using that you could comment on where you think oil or varnish doesn't produce what your system does.

Claudius, thank you very much for your reply. I was hoping that there was a publication somewhere that might provide further background material on the Tuscan varnish. I don't see much obvious variation in the varnishing technique in the pore photos that I posted. If you do, I am very interested in hearing more. (I should point out that two different cameras were involved and that my camera settings were likely very different for each photo. The overall colouration seen in the photos is not what actually existed. Some are worse than others but the overall colour is not at all reliable in any of them. All I was trying to do in posting these was to illustrate the way in which coloured material in pores can degrade. If you download the images and zoom in you can find pores where colour still exists at one end of that pore while at the other end this colour has broken down forming a black looking mass.) Thank you for sharing the photos that you have along with your explanation of each. What you mention makes sense. I particularly enjoyed seeing the Strad cello photos!

Claudius, I am interested in knowing the source of the second image in your post. An alternative version of this image appears in various publications but I have not been able to find where the particular version you have shared, with labelled areas A, B and C, has been published. I would be grateful if you could let me know where you sourced this image.

Owen, the below feature pore detail from five Strads from the period c.1696 to 1721. If you zoom in you should be able to see the sort of detail that I have referred to.

Yes it is a fracture surface. This was one of many samples looked at over five sessions between 1990 and 1992, so is some time ago. From what I recall I split each sample along the grain perpendicular to the varnished surface with a blade of some sort. This seemed the easiest to do. In my rush to prepare the various samples it seems that some were not as dry and hence stable as I might have imagined. It is interesting the extent to which the pigmented coloured layer ended up sagging and, as you note, even the clear over layer did something similar but to lesser extent. I can only imagine that I plastered on the coloured layer and when I thought it was firm enough, applied the over layer(s). At that time I was using equisetum to smooth the surface of some layers. In this case I could well have done this following the first coat of clear over layer following which a second more thinned coat was applied in an attempt to eliminate any scratches, scuffing or dulling left after sanding. There also could have been sanding/smoothing involved between the ground varnish and coloured layer, and coloured layer and clear over varnish. Unfortunately my notes dating from that time are not as good as they could have been. All I can tell you about this particular sample with certainty is what I have mentioned in the post accompanying the image.