## 4 Discussion  
Free oxygen vacancies are formed in YSZ ceramics during the SPS process, because of thermal action in a reducing environment [16]. The formed vacancies are centers of light scattering and have a negative effect on the optical properties. Thermal annealing of YSZ ceramics in an oxygen-containing atmosphere makes it possible to restore the oxygen stoichiometry and improve the optical properties [17; 18; 19].  
The high transparency values for the YSZ ceramics obtained in the present work are apparently related to the duration of the saturation process and the number of oxygen vacancies not recovered during heat treatment in the air. In our opinion, to increase the light transmission of high/bulky (>3 mm thick) ceramics with an increased shape factor, it is necessary to increase the saturation time of oxygen vacancies by increasing isothermal holding at a given annealing temperature or to significantly reduce the rate of temperature change to 10-50 \({}^{\circ}\)C/h.  
The presented approaches to optimizing the SPS modes for YSZ sintering ceramics may raise some doubts related to the accuracy of temperature control for the sintered ceramics. The radiation pyrometer, which recorded temperatures on the outside of the die, is typically used during the SPS process. The quantitative distributions of temperature fields in the die volume and in the sintered material in the SPS process are difficult to implement or simulate because they depend on many factors. The minimization of temperature gradients in the sample is possible only in the long-term (quasi-stationary) SPS mode that often leads to the growth of ceramic grains. In such a situation, the optimization of the SPS modes can only be carried out for specific products and dies. In this case, the transition to other sizes and dies may require re-optimization.  
Nevertheless, the optimization and correct quantitative comparison of the results using different pressing schemes in SPS with the same modes is possible with a good coincidence of the volume and geometric die dimensions. The possible deviations of the real temperature of the sample from the values recorded on the surface die will be close for the collector and conventional dies having the same volume and parts geometry. This makes it possible to carry out a quantitative comparison of the sealing surfaces and the coefficients of the corresponding pressing equations to optimize the modes in these specific conditions of consolidation. It also allows applying the proposed approach to comparative optimization on other plants and materials.  
It should be noted that for the same reason (closeness of the dimensions and geometry of the tooling), the use of a manifold pressing scheme and molds does not impose additional restrictions on the productivity and cost of implementing the SPS process. The use of a collector pressing scheme in SPS opens up prospects for the fabrication of products with complex shapes [11]. It favorably distinguishes this approach from compaction schemes of other widely used hot pressing options [20]. Moreover, the collector pressing scheme allows one to solve the well-known problem of limiting the prepressing pressure in the SPS process [16]. The conventional punches of SPS dies can transfer pressure to the sintered sample. The pressure is limited by the compression strength of the graphite. The load-bearing section of the pressing element can be larger than the section of the sample to be pressed using a collector pressing scheme. It makes it possible to apply a proportionally greater preprocessing pressure to it in a die made of the same material.  
The reasons for the anomalous increase in the transparency of thicker specimens made in SPS according to the uniaxial pressing scheme noted in the experiments are not yet clear. This issue requires separate experimental studies on YSZ and other ceramics, schemes for their consolidation under the SPS process, and atmospheric annealing modes.