1. Chemical composition of the Bohemian potassium glass
and the corrosion manifestation on excavated glass
1.-5. July 2013
ICT Prague
Dana Rohanová, Simona Randáková, Petr Bezdička, Aleš Helebrant, Olga Tomanová
Department of Glass and Ceramics, Institute of the Chemical Technology Prague, Technická 5, Prague 6, Czech Republic
Institute of Inorganic Chemistry of the Academy of Science Czech Republic, v.v.i., Řež -Husinec
Introduction
Potassium glass production is dated from the 13th till the 1st half of 19. century AD in Bohemia area. So called woodash glass (sand and beech ash) was not produced in Bohemia. Our forefathers produced woodash-potash glass very early (potash was leaching mainly from beech ash). Potassium glass production apparently came through the three different periods, which could be approximately connected with the architectural styles (gothic, renaissance and baroque). The glass melted at each period was different not only by shape. There are different also in raw materials used for batches and final chemical composition. Potassium glass has lower chemical durability than sodium glass, mainly because of lower SiO2/R2O ratio (R= Na or K). Gothic glass is often totally dealkalized or affected by pitting corrosion. Dulling and iridescence are typical for glasses from renaissance. The chalk glass produced in the baroque period (without ash addition) has higher durability and the corrosion in the form of light iridescence or dulling (cracking) was observed.
Chemical composition, average values, Chrudim`s glass [wt%]
SEM/EDS, XRF
Oxide
SiO2
K2O
CaO
MgO
Na2O
MnO
Al2O3
P2O5
Fe2O3
SO3 Cl
As2O3
K2O/CaO
Gothic
58.0
19.9
15.4
2.5
0.2
0.9
1.5
1.0
0.3
0.1 -
1.3
Renaissance
60.7
13.7
17.2
2.2
1.1
1.8
0.5
0.8
Baroque
74.8
13.4
9.6
1.4
Corrosion manifestation
Raw materials: SiO2 (sand, quartz), beech ash, potash, later CaCO3 (Ca(OH)2)
Gothic style (14th - 15th AD)
Conclusions
Total dealkalization
Total dealkalization
SiO2
Precipitation
Total dealkalization
SiO2
Precipitation of
Precipitation
Hydroxyapatite
Flaking
Renaissance (16th - 17th AD)
Cracking, Flaking off
SiO2
Iridescence
Dulling
Iridescence
Iridescence
SiO2
Pitting
Baroque style (18th - 19th AD)
Fe spots
SiO2
Pitting
Crizzling
Crizzling
Acknowledgments
Crizzling
This work was supported by GAČR : P 405/12/1411: Renaissance and Baroque Glass in Central Danube Region
Authors thanks for restoration work to students of the ICT Prague at the program: Conservation and restoration of historical objects from glass and ceramics :R. Kozáková, L. Klikarová, B. Hrubá, J. Seifertová, V. Kumstová, V. Šimková, T. Hüttnerová, T. Syrovátková.