Click on the pic to view close-ups of the sword fittings of the 2 Tang swords shown on this page >>>
Middle and bottom pics: 2 Tang Dynasty Chinese blades (around 8th century AD) in the Shosoin Depository at Nara, Japan.
Both exhibit straight distinct differential heat-treatment patterns that could only come about through using clay.
The 3rd blade exhibits the ridged "shinogi-zukuri" cross-section, but there is a difference, the thickness at the ridge is approximately equal to the width at the back of the blade, later periods' Japanese swords have a ridge thickness that is greater than the width at the back.
Sui and Tang Dynasty swords, like their Han Dynasty predecessors, were typically forged / folded and incorporating forge-welded/laminated construction, with ridged cross-sections featured as standard.
The differential heat-treatment then was to use clay as a form of thermal insulative material; this process of using clay for heat-treatment was developed sometime between 200BC - 500AD.
This was accomplished by coating a thicker layer of clay along the spine/back of the sword and a thinner layer of clay along the cutting edge. These 2 layers of differing thickness were then allowed to dry and the sword was afterwards heated up and quenched in water. The result was that the edge would cool faster than the spine/back, resulting in a hard edge and a soft spine/back.
After polishing, the sword would show a snowy white pattern (consisting of martensite crystals) running along the cutting edge, an obvious and visible result of the differential heat-treatment.
During the later Han to early Tang Dynasties, all these manufacturing techniques were adopted by the Japanese, which subsequently became the basis and foundation of the Japanese sword for the next 1200 years.
The Chinese influence could also be seen in the fact that up to the 12th century, certain Japanese swordmakers continued to regard Warring States swordsmith Ganjiang as the spiritual patron of their craft, a tradition that was mostly likely handed down by the Chinese/Korean swordsmiths who had earlier, migrated to Japan.