36.1 I am here too- When manganese turns into an enemy

15/11/13

For over 20 years C.T.S. has promoted and provided a unique product which is very popular among the restorers working on finds from archaeological sites. It is the well known B.D.G. 86, a ready-to-use neutral solution based on hydroxylammonium chloride and hydrazine (redox agents) and that is capable to clean stains of manganese.

It is very difficult to remove these kind of stains because they are very deep-rooted inside the porous materials. As a matter of fact bacteria and algae and fungi living on the substrates (for example the so called bacillus manganicus) can turn the metal (when it is present) into dioxides (pyrolusite) or hydroxides (manganite and psilomelane).
One solution for bleaching these minerals could be the adoption of the reverse reaction of the white lead, already studied by Matteini in 1976 [1] when dealing with wall paintings. That method takes advantage of the 'double nature' of hydrogen peroxide: when pH of contour is basic, the peroxide acts as an oxidizer, when pH is acid, it acts as a reducer. In order to avoid the effects of acid pH (due to the acetic acid) on the carbonates of the murals, the scientists and technicians of 'Opificio delle Pietre Dure' carried out an accurate study on the application times and on the related methodology by means of gelation of carboxy-methyl cellulose (CMC) that reduces the diffusion of reactants inside the porous substrate.
Here is the reaction:

PbO
2 +H2O2 + 2CH3COOH → Pb(CH3COO)2 + 2H2O + O2

In such a way, dark lead oxide (from the degradation of white lead) changes into white lead acetate (then, as time passes, again it turns into lead oxide through 
the carbonatation reaction).
The advantage of working on surface becomes a limit for cleaning the manganese stains in depth. Therefore, the restorers decided to implement a methodology that enables the object to stay in contact with the mixture with no risks of secondary bad reactions. Following an investigation carried out under the supervision of the Government Institution 'Soprintendenza Archeologica di Roma' [2], and using X-ray, scattering techniques, scanning electron microscope (SEM) and micro analysis, they could 
say the most effective substances are those belonging to the family of B.D.G. 86, whose active components are hydrazine and hydroxyl -ammonium.

The crucial 
parameters when choosing the best grade of B.D.G. are concentration and pH. For instance, if concentration is too high, it could damage the fragile materials such as pottery, bones and glass.
So, the concentration of 10% is suggested only for the most resistant ones such as stones. Light acid pH is recommended when treating glass and bones. Two formulations have been buffered to pH 5.5 and pH 6.5 respectively.
The table below shows the results:

material               grade of B.D.G.86               pH               active substance concentration (%)
  stony                        green                             7                                    10
ceramic                        red                               7                                     5
  bony                        yellow                            6.5                                   5
 glassy                       azure                            5.5                                   2.5


B.D.G.86 is applied by means of cotton wool pads placed upon the surfaces in some other cases by immersion of the artwork.
The operation takes 15~20 minutes and that is effective just on the tenacious manganese aggregates.
When the stains are strictly combined with calcite/ silicon/ phosphate based incrustations, the time required for the application is longer, up to 30~40 minutes, the maximum limit for avoiding 
the recrystallization of salts. 
Sometimes there are also reddish-brown stains of iron oxides. These cannot be cleaned with B.D.G. 86 [4]. In that case it is necessary to swab with di-sodium EDTA at 10~15%, from 20 up to 45 minutes. Immersion in EDTA at 10 % can work as well.
We recommend that, after the treatment, the operators wash the artwork with deionized water, we suggest using alcohol for iridescent glass-works and damaged bones.

Over the past years 
B.D.G. 86 proved itself to be a reliable product addressing the manganese related issues...we can mention several impressive case studies like that one of the identification of an important Etruscan find [5]. B.D.G. 86 is a 'day-to-day practice ' product in many laboratories dealing with archaeological finds [3][6]. For additional pieces of information about comparisons with different cleaning methodologies, please refer to the references [7].

Finally, we should mention a recent application for taking out black stains from the stucco decorations of the vestibule in the underground basilica of Porta Maggiore, Rome.
Other trials have been performed on the aisles and the apse with impressive results.
References:
  • Matteini M., "Ossidazione della biacca in pitture murali. Metodi proposti per la riconversione del pigmento ossidato nelle pitture di A. Baldovinetti nella Chiesa di san Miniato a Firenze”, publications of the conference on restoration of art works, Florence, November 2-6th, 1976, pagg. 257-269.
  • Bandini G., Diana S., Guidi G., "Rimozione di macchie nere da reperti ceramici, vitrei e ossei”, in Notiziario ENEA, nn.7-8, July-August, 1989.
  • Fancelli P., "Una scoperta nel restauro archeologico. Rimozione delle macchie nere da reperti di scavo”, in "Antiqua”, a. XIV, n.1, 1989, pp. 44-50.
  • Bandini, G., "Metodo combinato per la rimozione da ceramiche di macchie causate da composti di ferro-manganese”, in "Faenza”, a. 80 (1994), n. 3-4, pp. 117-121, tavv. XXXVI-XXXVII. Records from the second day of the meeting on restoration of pottery, Faenza, September 25th, 1993).
  • CarraroA.- Mizzoni F., "Cambiamento di immagine e restituzione di leggibilità del ‘Sarcofago dei Leoni’ nel Museo di Villa Giulia a Roma”,in "Faenza", a. 80 (1994), n. 3-4, pagg. 114-116, tav. XXXV. (Atti dellaII Giornata di Studio sul restauro della ceramica, Faenza, 25 settembre 1993). Records from the second day of the meeting on restoration of pottery, Faenza, September 25th, 1993
  • Banegas de Juan I., "Conservació-Restauració de material ceràmic arquològic. Laboratori de conservació-restauració del museu comarcal de l HYPERLINK "http://dialnet.unirioja.es/servlet/articulo?codigo=2659552"' HYPERLINK "http://dialnet.unirioja.es/servlet/articulo?codigo=2659552"Urgell”, in URTX n. 20, 2007, págs. 349-362
  • Venault de Bourleuf E., "Caractérisation du phénomène de brunissement du vitrail et évaluation de traitements de réduction” CeROArt [Online], |2013, online since 11 May 2013, connection on 05 September 2013. URL: http://ceroart.revues.org/3237

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