Protective Coatings for Medieval Stained Glasses
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PROTECTIVE COATINGS FOR MEDIEVAL STAINED GLASSES H. Schmidt* and D. Fuchs** *Institut fUr Neue Materialien, Gebdude 43,Im Stadtwald, Saarbricken,F.R.G. "*Fraunhofer-Institut fUr Silicatforschung, Neunerplatz 2, WUrzburg, F.R.G.
6600 8700
ABSTRACT The corrosion protective effect of the surface modification of corrosion sensitive glass surfaces and of ORMOCER (= organiThe surface cally modified ceramic) coatings is investigated. modification provides a modest protection, perhaps useful for The additional protection in combination with external glazing. advantage is the "invisability" of the modification due to the An effective protection thickness of several molecular layers. can be obtained by a diffusion barrier pigmented ORMOCER First outdoor results on medieval glasses are very coatings. necessary for wider hopeful, but long period experience is application. INTRODUCTION The corrosion of glass under the influence of H20 is a well known phenomenon. An ion exchange process takes place between a proton and an alkali ion, as a rule, according to (1).
uSi-0O Na÷ + H÷ - + NaOH
-
=Si-OH + Na÷
(1)
The sodium ion in an aqueous environment forms NaOH, which can lead to a network dissolution of the silica matrix (2).
uSi-O-Siu +NaOH
-
=Si-ONa + HO-Siu
(2)
sodium carbonate will be formed. In presence of C02 , Potassium containing glasses basically react in the same way. e.g., modern calcium hydrolytic stable glasses, Whereas in silicate glasses the ion exchange is mainly restricted to tho in glasses with high potassium and high alkaline ions [1, 2], calcium contents calcium ions are exchanged, too, as shown in [3] In these glasses with about 18 mole-% of by ESCA measurements. K20 as well as of CaO, in a corrosion experiment in humid air, K As shown in this investigation, as well as Ca is leached out. too, the leaching out of Ca is enhanced in the presence of S02. This can be explained by the formation of syngenite, a potassiumThis leads calcium sulfate, which is almost unsoluble in water. to a drastic decrease of the Ca2+ ion concentration in the diffusion path and to an increased driving force for the process in equation (3).
Mat. Res. Soc. Symp. Proc. Vol. 185.-01991 Materials Research Society
228
aSi-O-Ca-O-Sia +H 0 - =Si-OH + HO-Sim +Ca -0 - CaSO4 Ca 2 ++ so+ H20 + CaSO4 + K2SO 4 _+ K2Ca(SO4 ) 2 -H 20
2
÷ (3)
ESCA analysis including depth profiling shows that corrosion in presence of S02 leads to a much stronger decay in Ca concentration These in the glass surface than in a S0 2-free atmosphere. decades a that during the last findings explain the fact, dramatic increase of corrosion on medieval glasses was observed. Most of these glasses have high potassium and high calcium contents as well. of the The ion exchange with protons as the basic origin corrosion process at room temperature is not restricted to the liquid phase, but can take place with the humidity of the air. Due to an adsorption/desorption equlibrium on a glass surface, 0 water molecules are present up to 200 C (4).
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H
Si-O
H
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