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Patina Susceptible to Crumbling

Discussion in 'Copper & Bronze' started by SuchMuch, Apr 20, 2016.

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  1. SuchMuch

    SuchMuch Administrator Staff Member

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    Delamination of upper dense patina layer that includes relief details and being an original surface is caused by unstable, loose subsurface layer consisting of corrosion products. Therefore upper dense layers (of patina) are poorly connected to the lower layers (of patina) and this connection is very unstable.

    Let us review reasons those cause delamination of upper dense patina layer. As a rule oxygen and oxidation process take less part among the reasons. Thanks to lower cuprite layer the process of oxidation is very slowed down and once find is picked up oxidation is of little importance (when it takes short time between find is dug out and its conservation).

    From my point of view, there are several reasons those lead to patina crumbling and one of them is improper unstable patina handling:

    First of all, it's an improper find's washing aimed to remove soil residues present on the find. Unfortunately it usually isn't possible to find out at first sight whether lower layer is loose or not, but it is possible in most of the cases. If there are signs (chips, spots with absent surface dense patina etc.) that connection between upper and lower layers is loose then you should act very accurate. Do not scratch find's surface with fingers, stiff brushes and avoid using damp tampons. Do not place such find under falling water. If patina allows clean it with soft brush (SuchMuch's remark: I use soft brush from photo cleaning kit), use soft brush only in cases when it's really possible (totally no damage to the find). Get used to predicting object's condition. Sometimes microscope could help to recognize find's condition.

    Improper cleaning results could be observed on below picture. The target had thin unstable patina, it was cleaned with damp cotton tampon without any physical efforts. However major part of patina was lost:

    Here is another example of patina that seemed to be dense and stable at first sight. But that was not true. Once it was cleaned with damp cotton tampon, patina part was lost:

    Secondly, it's a wrong handling of fragile find right after it was taken from the ground. Usually find is in damp condition when it's dug out. This damp condition should remain until the process of washing begins. Find's drying should be avoided by all means. When patina is being dried 2 effects are observed:

    1. Corrosion products are getting compressed when find is being dried, should find becomes wet again corrosion products will expand. This effect could be observed when pool is evaporated and dirt dries and cracks.
    All this certainly occurs in smaller scales, but it has great influence to object. I must say aforementioned compression and expansion happen with many cellular structures. For instance, take sandstone or common red brick and wet them, brick and sandstone will increase their dimensions by 1 mm being just 15 cms long and such expansion is great. By the way, patina consists of layers with different physical and chemical qualities with expansion coefficients varying from layer to layer and this fact complicates the whole picture.

    2. As said above unstable patina layers have cellular structures. Lower loose layer has more efficient capillarity system if compared to dense upper layer. Should object be buried in damp soil this capillarity system is fulfilled with water and salts dissolved in it. When object dries these salts turn into crystals. Salt crystals' pressure is so great that any dense structure could be destroyed. Salt crystallization do not always lead to total destruction of dense structures (delamination of find's original surface). Upper layers are lifted by crystals and cemented with lower layers at the same time, thus patina turns into solid cover. Should water get into object's capillarity system, salts will be dissolved and delamination effect will occur.

    Preventive conservation

    1. Once find is dug out do not scratch it and avoid cleaning it for purpose of better identification. Handle find with care.

    2. Try to figure patina condition out and should dense patina layer be damaged (cellular structure, cracks and chips, cones and outgrowths, absent fragments) cover object with tissue moistured with distilled water (or just cover with ground sample taken from the place object was found at) and put it in hermetic box. Thus patina drying and occasional damages (due to vibration, shakes) caused by transportation will be avoided.

    3. Reduce time period between point when object was dug out and point desalting process (soaking in distilled water) was commenced. Avoid having find covered in moistured tissue for a long time otherwise there is a risk of corrosion. Especially this concerns objects with active bronze disease spots.

    4. In case object has dense stable patina remove ground residues with soft brush and distilled water

    5. All the objects with damaged patina (no matter delaminated or solid patina) are subject to be desalted. It must be mentioned, soaking in distilled water for a long time is no good for patina. It's important to estimate optimal time the find to be soaked (in distilled water). Finds with thin patina shouldn't be desalted in distilled water longer than 2 days (48 hours). If there is a chance patina could be damaged whilst being cleaned (with a soft brush), such objects are not cleaned, they are soaked (in distilled water) with ground residues.

    Once desalting process is completed there are 2 variants to conservate (fix) fragile patina:

    1. Completely dry under infrared lamp. Fix crumbling patina with acrylic polymer. Patina will be fixed within ground residues. Then we can proceed cleaning object with mechanical instruments.

    2. If object's surface is very fragile and there is a risk it could crumble whilst drying or it could crumble once dried object is put into fixing solution, damp find (right after the soaking in distilled water) should be dried by spirit solution. Concentration of spirit should be increased gradually in accordance with patina quality and character of pollution (E.g.: spirit concentration is increased by 25% each 12 hours). After that the find should be put in acetone. All these measures are applied in order to remove moisture from capillarity system.

    Once object is dried, all its fragile parts should be fixed with polymer (SuchMuch's remark: E.g.: Paraloid B-72). Then object is immersed into polymer solution (7,5%-15%), concentration of polymer solution is increased up to 15% if needed. (SuchMuch remarks: object is put in several solutions, first solution with the lowest polymer concentration goes). Avoid using tweezers (in order to prevent damage done to the object) when placing object from one solution to another, use strainer. This is the way all fragile parts (within dirt) are fixed. Once object is dried it's subject to be cleaned by mechanical means.

    Существует и другой вид патины – мягкой. При этом слой оригинальной поверхности (как правило малахит) не сформировался в плотную массу. При неосторожном механическом воздействии происходит осыпание этого слоя. В данном случае действия очень похожие. Ни в коем случае не расчищать поверхность. Вначале закрепить всё вместе с загрязнениями, и лишь затем расчищать.

    Визуально всё так и останется, удалять там больше нечего. Если мы удалим весь малахит, то под ним откроется «шершавая» неприглядная поверхность. Пропадёт часть сохранившегося рельефа. Вот например, буковка L после этого будет практически незаметна. Пропадут точечки в короне ну и т.п..

    Консервация подразумевает выявление формы предмета и дальнейшее сохранение его в том виде, в котором он есть. Другое дело реставрация. В случае с этом монетой, возможно заретушировать тёмный куприт (за пигментировать). Для этого подойдет тот же самый малахит. Это действие совершенно не влияет на сохранность, но так сказать улучшит внешней вид монеты.
    Sandandmud and Turbowrench like this.
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