Refining

Question 1

What is refining?

Answer 1

~the removal of bubbles from the melt

Question 2

What are the variations of bubbles?

Answer 2

~seeds: 0.5 mm

~ blister (larger)

Question 3

What are the requirements for automobile windshields?

Answer 3

~ less than one 0.2mm seed in a square meter of 3mm thick glass

Question 4

What is the ratio of evolved CO2 in window glass to the volume in the glass?

Answer 4

~ evolved CO2 is ~200x the volume of the glass

Question 5

What is the volume of trapped gas from voids in the solid batch relative to the volume of glass derived from batch decomposition?

Answer 5

~ volume of trapped gas from voids in solid batch is SMALL relative to the volume of gas derived from batch decomposition

Question 6

What is the velocity of a rising bubble based on Stokes law?

Answer 6

~ velocity of a rising bubble increases with the square of its radius and decreases with increasing melt viscosity

Question 7

What is the first step to refining?

Answer 7

~ melt is heated to a temp much higher than that four later pouring to facilitate refining

~ in addition to LOWERED melt viscosity, diffusion rates of soluble CO2 to a bubble will be high and SOLUBILITY of CO2 in the melt DECREASES with increasing temp

~ bubbles become ENLARGED from exsolution of supersaturated species, and with COALESCENCE, are REMOVED by floatation

Question 8

What is the second step to refining?

Answer 8

~ melt is cooled from max temp

~ increased CO2 solubility SHRINKS the bubbles

~ all bubbles tend to resorb when they become small enough for surface tension to increase internal pressure significantly

Question 9

What is the equation for change in internal pressure in a bubble?

Answer 9

~ Δp = (2*surface tension)/pore radius

Question 10

At what point do bubbles tend to collapse?

Answer 10

~ when they become smaller than 0.1mm

Question 11

What’s the effect of arsenic oxide additions?

Answer 11

~ batches become seed-free significantly faster

~ as temp is raised to lower end of melting range, As5+ is reduced to As3+, giving off oxygen that sweeps out fine seed, and leaves bubbles in the melt oxygen-rich

~ cooling the melt during refining period, As5+ will again be favored, acting as an oxygen sink within the melt, accelerating solution of oxygen-rich bubbles

Question 12

What element acts similar to arsenic oxide additions?

Answer 12

~ antimony

Question 13

When can Mo and graphite electrodes not be used in cold top melters?

Answer 13

~ when the melt has high levels of easily-reducible oxides (As2O3, Sb2O3, PbO, SO3)