Fumed Silica treatment can improve details in your Siderotypes, but be careful, it’s not chemistry without risk as Giorgio Bordin tells us.
Fumed Silica, as it’s commonly said, (CAS 112945-52-5) is also known as pyrogenic silica because it is produced in a flame. It consists of an extremely fine white powder made of microscopic droplets of amorphous silica (viz not crystalline), fused and branched in three-dimensional secondary and tertiary elements, in the order of nanoparticles (5-50 nm). A thixotropic behaviour (the property of changing its structure from sol to gel and vice versa under mechanical stress) allows to use fumed silica as a thickener or reinforcing filler.
Its bulk density is very low (50 grams can fill a 15x15x15 cm box), but the surface area is extremely high (filling an area of 50-600 m2/g, with a density of 160-190 kg/m3). Being a silicon dioxide it is a kind of quartz, even if as you can see, it doesn’t resemble a rock at all. As Christopher James recalls in his book (Christopher James. The book of alternative photographic processes”. 3rd ed. Cengage learning. 2015. P. 270), Howard Hefner, a physician, member of the researcher’s team of Sullivan, calls the Fume silica “smoke in the bag”.
Being inert and non carcinogenetic, it is harmless if ingested; it is even used as a filler in lipsticks and toothpaste. But don’t let your guard down, because the powder can be inhaled and produce silicosis, a respiratory disease, due to the trapping of silica molecules in the airways with further inflammatory reaction and thickening of the inner structure of the lungs. This is why an appropriate mask should be used when manipulating this stuff.
Being firmly adhesive to the substrate and because of its adsorbent power, Fumed silica improves the diffusion of the incident light and it’s used to improve inkjet print papers, allowing the employment of pigments dyes, rather than inks.
In techniques like siderotypes (e.g. Platinotype, Cyanotype, Kallitype, vanDyke brown print, Ziatype, Chrysotype, Argyrotype) it can be used to pre-treat the paper to improve the outcome.
It effectively adsorbs the sensitizer, thus reducing bleeding, and keeping it on the surface, with better optical results in the final image. Given that it fills spaces between paper’s fibres, light can be better diffused giving more reflectance; making an inert layer between the paper and the sensitizer minimizes the interactions with the physical/chemical properties of the paper (this can reduce unwanted, but also impair desired effects).
As a result fumed silica increases details, expands the tonal scale either in the shadows, or in the lights, giving a higher optical transmission Dmax. Brilliance and shadow details are mostly improved where the dry down effect is outstanding (Kallitype, vanDyke).
After the treatment, the paper is velvety at touch (be careful: don’t touch the printing area, to avoid damage of the print).
Some important aspects, to which attention must be payed:
- Some papers get yellow stain, so make your tests. Arches Platine, Bergger Cot 320, Hanemühle platinum rag paper work fine in my hands.
- A high absorbance means also that the volume of the sensitizer for surface unit should be increased and brush coating could be uneven. A rod or a puddle pusher could be better.
- The sensitizer layer is frail when still wet, be careful.
- With some methods/papers/physical condition of the working environment, silica coating can vary the printing speed. Christopher James (Christopher James, ibid. pag 267) talks about a “slightly lower” speed, while in my experience the treatment can enhance speed by reducing the exposure time. See the figures for some example.
Early patents for fumed silica in siderotypes date 1947 (Edward Jahode, in blueprints) and 1958 (Beeber and Gold). Jahoda says that “by this new method the prints are true blue (ultramarine)”. Apart from being ultramarine different from Prussian Blue, it is true that colors are more brilliant and saturated. Sullivan has revived the process, but still few people know it and use it.
Chemical suppliers sell fumed silica in huge packaging, the cost of which is unaffordable for a single user, but if compared with the weight, you will find it cheap. 30 grams – the available sizing from Bostick and Sullivan in the USA or from LabOldtech in Italy, where I live – are enough for a lot of prints at affordable price.
There are two main methods to coat the paper. One is a dry method, the other is wet. Whoever wants to check the wet (immersion) method can read Christopher James’ book (pagg 268-270). It reduces the hazard of the powder; but I find very easy to dry coat.
To make the long story short, just roll the powder with a foam roll over the paper. Period.
In detail: wear mask and gloves (it is not dangerous for hands, but it will excessively dry the skin and it is hardly removed by water). The roll should be of good quality, like those mini hot dogs foam roller for enamel paint.
Put the paper over a smooth working surface (glass, plexiglas…) and secure the sheet with some tape. Avoid airflow draughts and carefully put a teaspoon of Fumed silica for a 20×25 print area, over the sheet. Then start gently to roll the area and as the powder seems to disappear, increase pressure and speed. Roll over both directions, for a couple of minutes, or longer (it won’t be harmful). The purpose is to fill all the spaces between the fibres.
Finally, rub the surface firmly and slowly with a squeegee to take off the excess of silica. Don’t use a cloth or it will remove unevenly the silica from the paper.
Now you are ready to coat the paper with the sensitizer (as said above, better with a puddle pusher or a simple rod) and print as usual.
Start to use this and you will hardly go back to the old.
To watch is worth more than many words: a video about the management of fumed silica can be viewed here:
Fig. 1 and detail
Pure palladium print. Pretreatment with Fumed Silica (right) compared with no treatment.
Both prints have been made from the same negative, with the same chemistry (Pd 1ml + Fe(III)Ox 1 ml + Na2 0,3 ml; development potassium Oxalate 50°C, three Clearing bath: EDTAII – NaSulphite – EDTA IV), on the same paper (Arches Platine 300 g/mq) with the same exposure (20 min), in the same condition of RH (37%) and temperature (16°C).
Scanning has been made in 16bit, saved as RAW data and developed in Camera Raw™ with the same parameters in the whole picture, while has been “normalized” when showing the details.
The print pretreated with fumed silica has more density, wider tonal scale in the shadows and better resolution in the highlights.
Also, sharpness is improved (see detail) and the texture of the paper is less perceptible, allowing a greater smoothness of the image.
Fig. 2 – Roses
Vandyke brown print on Hanhemühle platinum rag paper. Pretreatment with fumed silica. Incomplete Toning with Gold tyourea (L. Clerc formula) to allow some split of tones (highlights cooler than the shadows). 20×25 cm (8×10”) fomapan 200 negative Pyrocath HD developed.
Fig. 3 – the Courtyard
Pure palladium print: 24 drops Pd, 24 drops Iron(III)Oxalate + 1 drop Na2
Hanhemühle platinum rag paper, preheated with fumed silica
15 minutes exposure, development in Sodium Acetate. Three clearing baths (EDTA II, Na Sulphite, EDTA IV) and final washing.
Of this print, the video about how to coat with fumed silica here:
Fig. 4 – three prints
Kallitype with and without fumed silica: in this case a consistent increase in print speed is evident, with the need to cut the exposure time of 25%.
Negative, Ilford FP4, developed in HC110: DR 1,5
Kallitype. Hanhemühle Platinum Rag Paper.
Sensitizer Fe(III)Oxalate 20% AgNO3 10% 1:1
Developer Sodium Acetate (fHenry Hall), toning with Gold Thiourea (L. Clerc)
left: NO pretreatment with Fumed silica: 80 seconds Exposure. DMax 0,96
center: pretreated with Fumed silica: 80 seconds Exposure. DMax 1,47 (overexposed)
right: pretreated with Fumed silica: 60 seconds Exposure. Max 1,27 (correct)
Fig. 5 and detail
Pure Palladium print from digital negative.
21×11 cm. Hanehmühle platinum Rag Paper.
Development Potassium Oxalate cold
Scanned with the same settings, to provide a comparison.
Top (or right in the detail) with fumed silica pretreatment
Bottom (or left in the detail) without fumed silica pretreatment
In the detail, compare the values of Dmax in the shadows and the readings on a bright area. Numbers speaks clearly.