## 2 Experimental Setup and Procedure
### Plasma Pre-Treatment  
For plasma pre-treatment, both surfaces to be cemented were exposed to a direct dielectric barrier discharge (D-DBD) plasma prior to cementing. Here, the investigated glass samples were placed between the electrodes of the plasma source and thus acted as dielectric barrier. The setup used for such pre-treatment is shown schematically in Figure 1.  
In comparison to atmospheric pressure plasma jets or torches, this type of plasma stands out due to different advantageous features: First, the plasma is ignited directly on the surface of interest (see Figure 1), i.e., the surface to be cemented in the present case. The plasma species required for initiating surface reactions are thus generated close to the area of treatment. Second, plasma-induced ultraviolet irradiation, which is normally absorbed by the surrounding atmosphere, can contribute to plasma-chemical near-surface mechanisms such as the decomposition of surface-adherent molecules. \begin{table}
\begin{tabular}{l l} \hline \hline
**Sample Denomination** & **Sample Preparation** \\ \hline No ageing & Storing at ambient temperature for 48 h \\ \hline \multirow{6}{*}{Tempering} & Storing at ambient temperature for 48 h, then: \\  & \(\rightarrow\) tempering at a temperature of 60 \({}^{\circ}\)C for 1 h \\  & \(\rightarrow\) removal from oven \\  & \(\rightarrow\) cooling down to ambient temperature \\  & (procedure was repeated 5 times in total) \\ \hline \multirow{6}{*}{Water bath} & Storing at ambient temperature for 48 h, then: \\  & \(\rightarrow\) placing in a water bath at room temperature \\ \cline{1-1}  & \(\rightarrow\) heating of water up to 65 \({}^{\circ}\)C \\ \cline{1-1}  & \(\rightarrow\) cooling down to room temperature \\ \cline{1-1}  & \(\rightarrow\) removal from water bath \\ \cline{1-1}  & (procedure was repeated 5 times in total) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Overview on the different kinds of storing of cemented samples  
Figure 1: Schematic of the experimental setup used for direct dielectric barrier discharge (D-DBD) plasma treatment of glass surfaces to be cemented. Third, the direct ignition on the sample surface results in the formation of high electric field strengths at the surface and particularly at roughness peaks, leading to an acceleration of charge carriers towards the surface. This mechanism allows a certain selective material removal and a fine polishing of the glass surface [34; 35]. Finally, electric current between the two involved electrodes is significantly limited because the glass sample itself acts as dielectric barrier. A D-DBD thus features comparatively low gas temperatures. In the setup used here, the maximum temperature heating of the glass surface amounts to approximately \(T_{\mathrm{max}}\) = 90 \({}^{\circ}\)C as determined in previous work [36], i.e., well below the softening point of optical glasses. The plasma source was driven by a pulsed power supply providing alternating sinusoidal high-voltage pulses with a repetition rate of \(f\) = 7 kHz. The voltage and duration of each pulse was \(U\) = 11 kV and \(\tau\) = 80 \(\upmu\)s, respectively, see Figure 2. As a result, the energy of each high-voltage pulse accounted for \(E\) = 0.17 mJ and the averaged power dissipated in the plasma was \(P\) = 1.19 W as determined using the Lissajous method [37]. This method is an appropriate means for the determination of the dissipated pulse energy, since the voltage, as well as the current, have high rates of change with an uncertain phase position. Prior to cementing, plasma treatment of the glass sample surfaces was performed for in total 10 s (5 s per surface) where the plasma process gas was argon at a flow rate of 4 standard litres per minute (slim). Such short treatment duration was chosen since the major fraction of a plasma-induced change in contact angle on glass surfaces is achieved after merely some seconds [38]. The treatment was performed in a static arrangement.