Papermaking Additives Cause Degradation

Symptoms

Siderotype processes using ammonium ferric oxalate as the sensitizer, such as New Cyanotype, New Chrysotype, and Malde-Ware platinum-palladium, have been reported by some users to suffer an occasional problem on certain papers: the so-called “blotching” phenomenon. The image pigment fluctuates in optical density in adjacent areas of the print which should otherwise be smooth and uniform; i.e. it displays fibrous, “woolly” or “cloudy” patches, lighter in tone, and quite absent from the original image. On looking through the sheet, blotching is often seen to correlate with fibre flocculation (Latin: Floccus = ‘a tuft of wool’) in the formation of the web, and it is most conspicuous with those printing-out processes that are capable of very fine tonal separation in their upper values. This defect is highly dependent on the paper, and is therefore attributable to some non-uniformity in the sheet being disclosed and visually amplified by the very sensitive and responsive printing-out photochemistry of these processes.

Diagnosis

Image blotching is not mentioned in historical texts, but appears to be a recent problem — arising since the universal change in commercial papermaking in the mid-1980s, from acidic to alkaline chemistry. Today, it has become standard manufacturing practice to add so-called “retention aids” to the pulp at the wet end of the mill, in order to improve the trapping by the wire screen of fine particles of fibre, fillers, and other additives — this is generally done for commercial motives of lower machine running cost, efficiency in drainage, and easy water recycling.

Excessive use of retention aids is known to exacerbate flocculation, and these additives are likely to cause image “blotching” for the following chemical reasons. Retention aids are organic polyelectrolytes of very high molecular weight, such as cationic (positively-charged) polyacrylamides (CPAM), or cationic starch; they consist of long polymer chains bearing many positively-charged groups (quaternary alkylammonium cations) that will exert a strong attraction on the small, mobile, negatively-charged anions present in ammonium ferric oxalate photosensitizer chemistry, especially [Fe(C₂O₄)₃]^3-, [Fe(C₂O₄)₂]^2-, and [Fe(CN)₆]^3-, or [PdCl₄]^2- and [PtCl₄]^2-. These anions are liable to be trapped by local concentrations of the huge cationic polymer and rendered less reactive, thus wreaking havoc with the sensitive printing-out photochemistry. The papermakingbible.co.uk: “Adding starch to the pulp storage chest of the machine acts like a fixative. The cationic starch binds to the anionic trash in the pulp, cleaning the water loop, improving wetend retention and in some cases acts like a dry strength agent.”

(Retention aids will interact much less strongly with the oligomeric sensitizers of traditional development siderotype processes, like ferric oxalate or ferric ammonium citrate, whose linked molecules carry little ionic charge; this explains why the historic formulae like classic cyanotype and development platinotype don’t generally suffer from blotching.)

Rationale

A crucial test of this hypothesis would be to determine whether retention aids are always present in the afflicted papers but, regrettably, papermakers themselves tend not to be explicit concerning the chemical content of their products. We can only be guided by an authoritative review of retention aids which has stated that: “There is hardly a single paper mill in Central, Western or Southern Europe that does not use retention aids.”

Contrariwise, we do have the personal assurance of one craft papermaker that retention aids were always totally absent from his ‘Buxton’ and ‘Herschel’ papers, previously handmade at Ruscombe Mill, which have never suffered from blotching with any process: a case of proof by absence.

Possible Cures

With papers intended for siderotype processes, the likely cause of blotching could be eliminated at source by papermakers abstaining from the addition of retention aids to the pulp at the wet end of the mill. Until that longed-for day, we can only hope to resort to a partial “cure” for the papers currently afflicted with this disorder, by adding to our sensitizers a high concentration of an ionic salt that tends to disable the prejudicial action of these cationic retention aids by neutralising them with an excess of mobile anions. For reasons of solubility, compatibility, and humectant action, one such suitable salt is ammonium nitrate, NH₄NO₃. This can simply be added as solid to a ready made-up solution of New Cyanotype sensitizer, and easily dissolved so that its final concentration —which is not critical— is ca. 15-20% w/v (1.87-2.50 molar), i.e. ~2 g of ammonium nitrate should be dissolved in every 10 ml of sensitizer solution. Alternatively, the same sensitizer, which I will refer to as Anionic Cyanotype, can be made up from common chemicals, where the excess ammonium nitrate (2.12 molar) is formed safely in solution as a by-product of the chemical reaction.

Coincidentally, the addition of ammonium nitrate to improve the stability and quality of blueprint paper sensitizer was actually the subject of a US patent (no. 2,113,423) as long ago as 1938; although at that time retention aids were unknown, so it must have other beneficial effects as well. It is probable that the presence of ammonium nitrate in the coated sensitized layer, which is still consequently somewhat hygroscopic, will assist print-out of the image by maintaining humidity within the paper, even when the ambient Relative Humidity is low. My Simple Cyanotype sensitizer (of 2019) also contains excess ammonium nitrate as a consequence of the method of preparation, and seems to perform smoothly on several papers.

The Malde-Ware printing-out platinum-palladium sensitizers for Methods 2 and 3 are deliberately formulated to contain 17% w/v ammonium nitrate (2.1 molar) in excess, and are observed to be much less susceptible to blotching on certain papers than the Method 1 sensitizer, which does not contain it.

Damage by Alkaline Buffers

To conform to International Standards, papers intended for archival or fine art use are now required to be buffered to an alkaline pH, usually with added chalk (calcium carbonate), and they are commonly so advertised. But it is now widely acknowledged among alternative photography practitioners that papers intended for siderotype should contain no chalk, which is hostile to the iron(III) chemistry, its alkalinity causing hydrolysis of the Fe(III) complex; moreover, with cyanotypes it is directly destructive of the Prussian blue image substance, which is rapidly bleached at pH 9. Wise printers will employ one of the very few fine papers especially intended for siderotype, which are now manufactured commercially without added chalk; but many printers still resort to pre-treating alkaline buffered fine art papers with dilute acids, such as hydrochloric or sulphamic, in order to destroy the chalk before coating with sensitizer. It is possible that this ‘decalcification’ treatment also brings an added hidden benefit: the small anions such as chloride or sulphamate from the acid will tend to neutralise the cationic polyelectrolytes that are present as retention aids, unacknowledged, and thus may proof the treated sheets against blotching.

Effects of Surface Sizing

Thus far, we have only considered the eventual effects of additives to the pulp at the ‘wet end’ of the mill, before formation of the paper sheet; especially cationic retention aids and alkaline buffers, which are both totally inimical to siderotype chemistry. However, blotching can also be brought about by chemical treatment of the sheet after it is formed, i.e. by surface sizing, whether applied in the mill or subsequently by the user of the paper. Surface sizing practice is usually carried out by immersion or flotation in a bath of a hydrophilic film-forming colloid, most commonly gelatin or starch. Hardening agents, which are often chemical reductants, may also be added. For siderotypes there are possible objections to the use of both these surface sizing chemicals. Gelatin-sized papers must be avoided If the sensitizer contains a significant proportion of platinum, because gelatin can bind strongly to platinum(II) in aqueous solution, and the resulting protein (aminoacid) complex of platinum(II) is less readily reduced to platinum metal by the iron(II) photoproduct. This problem does not arise with palladium, which is more reactive. Some practitioners continue to recommend the use of gelatin sizing, because they think that it appears to work with their particular modus operandi, which usually involves a mixture with palladium, and they may be fortunate in their choice of gelatin, which at the present time is likely to be a more highly-refined "food grade", unlike the crude "animal glue gelatins" of the 19th century used for sizing papers, which were scrupulously avoided by William Willis for his Platinotype paper.

In papermill practice, surface sizing with starch does not employ the usual familiar substance of that name, but rather a derivative called “oxidised starch”: this product of treating starch with either hydrogen peroxide or sodium hypochlorite as oxidants, has reactive functional groups, especially carboxylates, which can bind the iron(III) (as indeed they do in ferrioxalates and ferricitrates) and thus diminish the efficiency of the sensitizer, shortening the exposure scale, and possibly sequestering iron(III), which eventually causes staining. In general, no good purpose is served by starch-sizing a paper intended for siderotype processes. Papers internally sized with alum-rosin or alkyl ketene dimer (e.g. Aquapel) seem perfectly compatible with the processes.

Mike Ware

Buxton, November 2020

Paper for Siderotypes

Summary Specification

Furnish nearly ~100% alpha-cellulose
- Either long-fibre cotton or linen, but not mixed
- Weight ~160-240 gsm usually adequate
Wove mould – not laid – no watermarks in picture area
Absolutely NO additives of :
- Alkaline buffer - Chalk (calcium carbonate) or other alkali
- Clay filler or aluminosilicates
- Retention aids – cationic polyelectrolytes such as CPAM (cationic polyacrylamide) or cationic starch
- Wet strength aids such as Kymene PAE (polyamidoamine- epichlorohydrin resin)
- Optical brightening agents OBAs
- Dyes or white pigments such as titanium dioxide
- No residual bleach
Internally body-sized with AKD (alkylketene dimer) such as Aquapel
- Alum-rosin body sizing also acceptable
NO surface sizing, such as gelatin or oxidised starch
- No hardening agents (aldehydes)
Heavily cold-pressed or calendered surface