Glaze Firing Breakthrough
The bisque firing had been about driving out the moisture and organic matter buried in the clay. Glaze firing, on the other hand, was the stage where the glaze melted and coated the porcelain surface in a glassy sheen. Unlike the bisque, which could be done with firewood, this required an overwhelming temperature of 1,200 to 1,300 degrees. That meant using charcoal instead of logs. The kiln itself would take a massive beating at those temperatures too. The current setup, without firebricks, was a hundred percent guaranteed to melt down. Still, if I faithfully applied the refractory coating before each firing, it wouldn't collapse in one go. For now, that would have to do.
Each glaze firing consumed about 60 kilograms of charcoal—roughly the price of five silver coins. Factor in the bricks that would melt and need replacing, and every attempt was like burning a fortune. Right now, it was just practice with jars, but with costs this high per batch, from the next one on, it made sense to fill the kiln with high-value items relative to their size: small white porcelain dishes or teacups. Calculations showed I could fit around 100 plates or bowls, or even 300 small teacups. Even if dozens failed, as long as enough succeeded to sell, I'd start turning real profits.
I rested my chin on my hand, peering into the empty kiln. "……Looks like I'll need to make some firebricks after all."
Clay, who was peering in beside me, tilted her head. "Is there a problem?"
"If we go ahead like this, the supports will melt and collapse."
Like with the bisque, I'd planned to stack the porcelain two levels high inside the kiln, but now the regular bricks and tiles caught my eye. If they softened mid-fire and the jars toppled, everything would shatter. Good thing I'd noticed before lighting up. No choice—I'd use the settled clay from the sedimentation process to make a few refractory tiles for stacking and firebricks for supports. The mix: 70% kaolin, 20% feldspar, 10% silica. Unlike the porcelain body, I'd add about half the silica's volume in charcoal powder. Mixing it into the brick dough created pores as it burned away during firing, boosting insulation and thermal shock resistance.
Thinking ahead to the smaller pieces for future batches, I'd need to stack the kiln five or six levels high, so I made extra firebricks and tiles. I ground the materials fine on a millstone, mixed them dry, added water to knead, and let it age. Unlike regular bricks, aging the dough was essential here—firing cracks meant no recycling, and the materials cost more. Aging dropped the defect rate from 15-20% to under 3%. With the money on the line per firing, it was non-negotiable.
Combining aging, forming the bricks and tiles, and drying took a fortnight. Once dry, I used the refractory bricks and tiles directly as kiln supports. Any extras I left out—no need to cram them in; they'd serve as supports next round. Firebricks fired at 1,250-1,300 degrees, almost the same as the glazed jars. Running the kiln like this would fire them right alongside.
I nodded at Clay. "Let's start."
"Yes!"
We loaded charcoal and lit the fire. This time, fire for about 10 hours, seal the firemouth and chimney to kill the blaze, then cool gradually over three days. The charcoal glowed red inside, sucking air in with terrifying force. Heat blanketed the whole workshop. Clay wiped sweat from her brow as she added more fuel, and the chimney shifted from white smoke to blue flames laced with gray.
As quitting time neared, we sealed the firemouth and chimney. The next day, I cracked them open a bit on arrival, then a little more before leaving. Seemed minor, but rapid cooling cracked porcelain—it was crucial. On the third day, we fully opened the kiln.
Stepping inside to check the pieces, I let out a whoop. "Yes!"
Luck was on our side—no breaks in sight. I pulled out the jars and inspected them at length. Smooth, glassy interiors brought a grin to my face. I showed Clay one where the glaze had run like water droplets. "If you apply too much glaze, this happens. Be careful when glazing."
"Got it!"
Checking the kiln interior post-glaze, chunks of refractory coating had flaked off, cracks spiderwebbed some bricks, and a few were warped. Still, just a fraction of the total wall area. It'd hold another firing or two, I hoped. I reapplied white refractory coating meticulously, sprayed water to clean, and scrubbed. Leftover dust or ash could stain the next batch's surfaces.
With cleanup done, the surviving jars went straight into sedimentation for reuse. Then, prepping the next batch: white porcelain body at 6:3:1 kaolin-feldspar-silica, aged. Using the manor tableware as reference, I formed 100 each of bowls, plates, teacups, saucers, and small cups, bisque-fired them all. Defects: 2-5 per type, total 20 breaks.
Besides the usual 1:1:2 transparent glossy glaze (kaolin-feldspar-silica), I prepped more: 3.5:2:4.5 milky white glossy, 1.5:3:5.5 celadon glossy, and 5:1.5:3 milky matte with 0.5 wood ash. Four glazes, 25 pieces each per type across the 100 bisque items per form. Testing everything I knew, since who knew what would sell. Transparent glossy went last, so fewer due to breaks.
Post-drying, I wiped the kiln feet (the contact points with the shelf) meticulously with a wet cloth. Leftover glaze there could fuse to the shelf on glaze firing. With feet clean, time to load: 25 each of larger bowls and plates, 50 each of the rest. 200 pieces total, spaced about 1 cm apart, stacked neatly.
Ready. Heart pounding, I directed Clay. "Alright, Clay—light it up. We're starting."
"Yes!"
This firing's success meant everything. We'd poured in materials till we were nearly out, and the kiln walls would likely need full rebuild after. Failure could delay months. Same process as before; three days cooling later, we cracked it open cautiously.
And—
"Whoa! Young master, look! Not a single break!"
Clay, checking inside first, shouted gleefully. To inspect properly, we unloaded everything. Close examination: zero cracks. Maybe because they were small wares—or bisque had sieved out defects. No decorative incisions on bisque either, for stability. Whatever the reason, jackpot.
I ground the feet smooth on cooled pieces and washed them clean. Under the sun, transparent glossy, milky white glossy, celadon glossy, and matte milky white porcelains gleamed brilliantly. A few had glaze flaking or pinholes, but sellable flaws. Hey, unbroken is unbroken. Four months of work with Clay, paying off.
Kiln check post-unload: as expected, wrecked. Cracks everywhere, warped bricks beyond counting. One more fire, and the inner wall would collapse, raining molten brick. Regrettable, but better rebuild than cheap out and disaster. For now, finished wares first.
I rushed to the general store with horse and cart, bought heaps of straw. Back at the workshop, I wrapped each piece lovingly in straw, then layered the cart and loaded carefully. This'd survive transport.
With product in hand, time to report to butler Hermann at the manor and plan next steps. Village-made goods meant settling taxes too. Quick ride to the manor; at the gate, I had a servant fetch him while pulling the cart.
The servant eyed the straw mound glinting porcelain, said nothing, and dashed inside. Moments later, Hermann emerged, glanced at me, then the carefully packed load.
I pulled out the best teacup I'd saved, offering it. He peeled back the wrapping suspiciously. A milky white glossy beauty emerged; his eyes narrowed in disbelief.
"You…… really made this, young master?"
"Technically, Clay did."
His voice thick with shock, I flashed a confident grin. "What do you think? These should fetch a good price, right?"