MITRA Forum Question Details

​Regarding the clay, it depends on where you live and what portion of the clay is organic matter which might break down. Some soil has a lovely color when freshly dug due to plant material, but the organic components might not prove durable. Clay that is stained with oxides and ochres will be more reliable over time. As far as the walnut ink is concerned, any vegetable-derived colorant can be vulnerable to color change from exposure to UV light (though anyone who has hulled black walnuts may find it difficult to believe that the stain will ever fade willingly). If I were offering such a piece as you are describing to a collector, I would suggest display under protective glazing, out of direct sunlight or intense illumination, and recommend that the piece be stored at intervals, to reduce total exposure. This is really good advice in general for watercolors where some pigments are not of the highest lightfastness.

​I'm afraid I can't even hazard a guess about boiling or otherwise refining clay. I like your idea about using pigments from terrestrial (mined) sources. Some historical colors have gone extinct as mines have become played out, (the original Venetian Red, Caledonian Brown, Cappagh Brown, and some other ochres were mined until the source was exhausted) but there are still some really great pigments that are not synthetic.

The salt will remain reactive to changes in RH but really, your piece should be stored in a stable environment anyway. I have seen many works where there are effects that had to been created using rock salt and they at least initially appeared in fine condition but that is not a good test for future longevity. I would think that it would be good to brush off what you can before framing etc. I will ask a paper conservator to comment on what NaCl does to cellulose.

There are a couple of transparent watercolor grounds on the market. I have to admit that I do not know what such a ground would be composed of. I am assuming it is some transparent filler like fumed silica or glass platelets etc., leanly bound in an acrylic or PVA dispersion. Without more info, I cannot really comment. Perhaps others here have a greater knowledge of these grounds.  

​If you are using a watercolor ground as a wet adhesive to stick paper to paper, if the ground is acrylic-based, it will likely work. A simple adhesion test conducted using scrap paper will reveal whether it will bond strongly enough. Gloss acrylic mediums are usually the best adhesives. Matting agents like the ones Brian listed as possible ingredients interfere with some of the like-to-like adhesiveness of acrylic products, but if the ground will stick permanently to one sheet ofpaper, it should stick two pieces together.

I received this for Joan Irving Senior Paper Conservator of Winterthur Museum & Affiliated Associate Professor of the Winterthur and University of Delaware Art Conservation Program. This really is a very good question – and I can’t recall anything in the conservation literature about this topic.  As we discussed, the main problems with the NaCl might be more theoretical than actual. If the watercolors are kept in a stable environment and in acid-free enclosures (mats, folders), they will probably be fine. Theoretically, the hygroscopic nature of the salt could raise the moisture content of the paper (with potential for foxing, mold, etc.). The other theoretical downside would be possible disassociation of the Cl ions, which could yield degradation products such as hydrochloric acid. However, most watercolor papers are really robust – generally quite thick and very well sized; sizing provides lots of chemi-mechanical protection and reduces the porosity and susceptibility of the paper to atmospheric pollutants, moisture, mold, etc.

So, if any paper could tolerate common sodium chloride – it would be a well-sized, rag, watercolor paper. It would be helpful to mat these objects in good quality, 4 or 8-ply, alkaline, 100% rag boards with zeolites (which are desiccants as well as good scavengers for atmospheric pollutants).

​In my experience, paper bonded by a wet-applied acrylic product (as opposed to heat fused) will remain bonded indefinitely if initial adhesion is good.