Once the lid of the autoclave is closed, the inside cannot be seen during the three to six month growing period. Thanks to years of know-how and master craftsmanship, NDK is able to assess the state of growth and ensure that the quality of the product is exactly as required, even though the inside of the autoclave is not visible.
The growth of crystals is caused by temperature differences in the solution in which the materials are dissolved. The temperature difference of the solution cannot be precisely controlled as the heater deteriorates, so at NDK, we made improvements to it through renovation work in 2017. The temperature of the solution is estimated from the temperature difference on the autoclave surface, but this method is not sufficient. So, in that same year, we devised a new method for predicting the growth rates of the crystals.
With this method, it has become possible to indirectly visualize the growth of crystals inside the autoclave. The growth rate is an important process that affects the "infrared absorption (α3585)" quality and frequency-temperature characteristics of synthetic quartz crystals.

In addition to the seed crystals, another factor that determines quality is the stability of convection of the solution inside the autoclave. At NDK, we use convection control plates (baffles) with independently developed structures for each growing specification to control the convection of the solution according to the required quality of synthetic quartz crystal.

High quality synthetic quartz crystals cannot be grown just by using sensors and controls.
Each autoclave has its own exquisite peculiarities, and it is up to the experience and skill of the craftsman to grasp them and adjust the temperature and convection. The quality of the product is greatly determined by the accuracy of the fixing of the lids, as the pressure inside can reach as high as 1,500 atm. They must be tightened with an accuracy of 1/100th of a millimeter, and this process cannot be mechanized because it requires subtle human manipulation.

An autoclave is a so-called "pressure cooker" that is used at high temperatures and pressures. Therefore, if the precision of maintenance is inadequate, it will not be able to withstand the high pressure conditions, causing the filled solution to leak out from the sheet area. If the solution leaks, the internal pressure drops, which may result in the suspension of growth, and in the worst case scenario, the leak will cause a break in the sheet area. At NDK, autoclaves are maintained only by workers with extensive experience in maintenance techniques, and as a result, there have been no leaks from the sheet area for several years.

The completed synthetic quartz crystals are inspected in-house according to the quality standards set forth by JIS.
The inside of the quartz crystals is visually inspected by permeating the gemstones underwater and shining light on them to inspect for inclusions, etch channels, etc. In addition, for the alpha value (Q value), a sample is cut from the specified target rough stone and measured in-house using a measuring instrument. (In some cases, we may request an external institution to assess synthetic quartz crystals with specifications that require impurity analysis (such as high-purity).)