This post shows the experience after the restoration of many of the architectural drawings from the Association of Architects of Catalonia (CoAC) and focuses on impregnated paper. It was presented on the context of the Symposium Technical Drawings and their Reproductions with the title “Replacement of varnishes/oils on impregnated paper architectural drawings” (The Hague, october 2014).
All tracing paper, so commonly used in technical drawings, have in common its translucency. But significant differences in its elaboration process provide them with diverse chemical and mechanical properties and behaviour. The so called impregnated paper is one type of tracing paper in which a varnish or oil was added in order to bring transparency.
Impregnated paper has poor mechanical strength due to the oxidation caused by the varnish. It must be reminded that oils do not evaporate by a physical process, like water, but a chemical one (oxidation). The most commonly used varnishes suffer from inherent oxidizing degradation that causes brittleness and acidification, but varnishes also prevent cellulose from moisturizing normally (waterproofing) and confer the paper its particular fragility and stiffness (shellac, especially).
The fragility may be such that it completely impedes manipulation, since any ordinary and careful handling causes cracks or tears. Loss of flexibility causes sharp cuts on the folds (left image) and tears on wrinkles (right). Tears and cracks can be even more visible than the drawing lines.
The goals to achieve with the restoration treatment are:
- Stop oxidation, or even revert some of its consequent acidification.
- Retrieve lost flexibility.
- Strict maintenance of scale (in case of plans/maps).
Consolidation by addition of paper layers compromises a transparency that shouldn’t be modified, that’s why retrieving the lost flexibility is so important, as the less brittle is the original paper, the thinner paper layers are needed to reinforce it.
Yellowing is not considered a damage in itself, as in general it allows a correct lecture of the document, and doesn’t involve chemical damage (yet it is the visible effect of acidification).
Removing the varnish
This would avoid further oxidation damage and diminish some yellowing. The varnish is removed by solvent (varying each case, depending on the oil or varnish to be removed). After that, the paper becomes opaque, but then efficient aqueous treatments are possible, if suitable.
Giving back transparency
Whereas using the original products seems not recommendable because of its visible detrimental effects, many options are still possible: Synthetic resins can be applied to both aqueous and non-aqueous procedures.
Anyhow, these resins would bring the transparency back and also the colour intensity. Yet this step obviously provides waterproofing too, the varnish will not produce any more ongoing oxidation, and it enables enough flexibility to handle the document safely.
|Innocuous||Transparency||♣♣♣||♣♣♣||Only limited by the addition of paper layers (not by the varnish).|
|Intense colour tone||♣♣♣||♣♣♣|
|Scale maintenance (architectural drawings!)||Must consider aqueous treatments each time. Anyhow scale can be strictly maintained even when applying wet treatments (previous test needed!).|
|Yellowing||♣♣♣♣||♣♣♣||Depends on the color and concentration of the original varnish, but also on the degree of oxidation, which may have yellowed fibres too.|
|Detrimental||Brittleness||♣♣♣♣||♣||Now a normal handling is possible without risk.|
|Acidity||♣♣♣♣||♣||pH increase in 2 points avg., reaching neutrality, when wet treatment is possible|
|Ongoing oxidation||♣♣♣||–||As the cause of it (oil or varnish) has been removed, the oxidation is stopped.|
Historical Archive of the Association of Architects of Catalonia (CoAC); owner of all the plans above mentioned and who trusted me for their restoration. Many thanks!