[Alt-photo] Outgassing in contact-printing siderotypes

Jorj Bauer jorj at jorj.org
Mon May 19 13:06:53 UTC 2014


I've run across this a few times while making many prints at once, some winding up more and some less dry than normal. Sadly I don't know which direction had more affect, as I wasn't keeping good track of which were sitting for how long.

-- Jorj

> On May 19, 2014, at 6:16 AM, Mike Ware <mike at mikeware.co.uk> wrote:
> 
> Regarding siderotype (iron-based process) contact-printing, there seems to be a technical issue, previously unrecognised, which I would like to share and invite comments.
> 
> During the exposure of all siderotype sensitizers - no matter which process - there is an evolution of carbon dioxide gas, which needs to find a pathway to escape through the back of the paper. 
> Otherwise it forms bubbles trapped between the negative and sensitized paper surface; a simple calculation suggests that the resulting gap can become large enough locally to blur the acutance or resolution of the contact-printed image. This is confirmed by my tests.
> 
> These "soft spots" are only conspicuous with a diffuse 'light bed' like a solarium, but not with an almost 'point source' like the sun or a NuArc unit.
> 
> In the contact-printing frame "sandwich", the sensitized paper should be backed with a porous felt blanket of the kind used by papermakers, or with other sheet material which is permeable to gas.
> If it is backed with an impervious rubber or plastic sheet (as I have - to my shame! - previously recommended, because I was concerned to retain humidity), then the CO2 will have nowhere else to go.
> Fortunately, several sets of instructions on the Web for making contact-printing frames specify a pressure backing of felt, so that many folk are doing the right thing, anyhow.
> 
> Another important ( - and hitherto unacknowledged?) factor influencing print quality could be the gaseous permeability and thickness of the printing paper substrate itself, which can provide some 'buffer volume'  for the evolved gas. Gelatin sizing of the paper may block the pores.
> 
> To summarise, the following factors (roughly in descending order of importance) will tend to degrade resolution by increasing the effect of trapped CO2 gas:
> 
> A diffuse light bed (solarium); impervious print backing; exposed borders; dark images; thin papers; surface ('tub') sizing with film-forming colloid; parchmentized papers ('vellum').
> 
> The following factors will tend to remedy the loss of resolution caused by outgassing:
> 
> Nearly a point source (sun or NuArc); permeable print backing sheet; masked borders; high key images; thick papers; internally ('engine') sized paper.
> 
> Results with vacuum easels may depend on the design: some (like the NuArc 2125, with a rubber blanket) will trap the gas, while others may pump it away.
> 
> I regret not thinking of this issue and addressing it 30 years ago! My only consolation is that during the 172 years of iron-based printing - as far as I know - there has never been any mention of the CO2 or where it goes to! It's possible that other printers may also have noticed blurring in contact-printing siderotypes, but mistakenly attributed it - as I did originally - to inadequate pressure in the design of their contact-printing frames.
> 
> Practices ... Experiences ... Comments?
> 
> Mike
> 
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