This study gives many new informations and complete previous
works [21,22] about ionic exchange process. First we show by several characterization techniques that silver or copper staining of a classical
silicate glass obtained for T>660 °C up to the glass transition temperature
favors combined conditions of exchange, neutralization, precipitation
and dissolution of the introduced noble metal. The elementary
analysis of the glass cross section presents in Fig. 7 shows a non typical
diffusion profile shape of copper and silver consisted of a known
diffusion law (in the range of 0 to 80 μmfor Cu and 0 to 12 μm for Ag)
added to a deeper rediffused population (located in the range of 80 to
120 μm for Cu and 12 to 24 μm for Ag). The exchange is balanced in
both cases because one sodium ion is substituted to one silver or
copper ion (Fig. 4). The evaluation of both ion diffusion coefficients
reveals that DAg>1.83.10-3 μm2/s and for copper, fifty more. These
differences are attributed to the unknown concentration of the deposited
enameling, an ionic radius and a higher mobility for copper.
Another explanation should be the possibility for some silver (a non
identified low rate) to travel deeply inside “adapted channels” of
the glass, because its diffusion coefficient looks too low in comparison
to a previous experiment made at 320 °C [26]. We know that Pauling
electronegativity gives χ for Cu (1.9), Ag (1.93) versus Na (0.93). The used commercial glass has Tin on one face due to the«float»process.
This particularity suggests this element can favor the precipitation
of silver or copper as follows and the formation of NPs limiting the
ionic diffusion [18]: