the affect of adding CaO to glass

[random_lexis]

Im an intern working for a comapny looking at the addition of inorganic material to glass. I am currently looking at the addition of CaO to cullet. The unaltered cullet has a suficient melting point at around 800 C, but adding even 5% CaO increases the melting temperature of the glass. On adding 50% material it gets to the stage that the CaO is vapourising before the glass is melting. Why is this?

I need to create a solid melted non porous pellet containing between 40 and 60% inorganic matter so im having trouble. to counteract the melting temperature problem i have tried adding both flux's and frits the flux ive tried thats been the most successful is B2O3/LiO2 which stops the clumping together of the inorganic material (onther problem) but also causes bubbling (not good) will the addition of sodium sulphate or chloride help with this. or is there a better flux i can try that will reduce the melting temperature, i would like to form the pellets at around 950 C. in addition to the flux i have tried adding frit, but i am also constrained by costs (hence the use of cullet) and so can only use low cost frits (so far i havent been successful) can you suggest any or a better way of approaching things?

[Greymatter Glass]

...as much as I'd like to help you.... you ARE aware this is an artistic glass forum, and very few of us have any experience with furnaces, and then it's mostly just remelting cullet with no modification.

I can sort of offer you some suggestions based on semi-educated guess work....

CaO will raise the melting temp of the glass, yep. I don't think there's any way around that.

Fluxes can only help so much before you get other problems.

You can make your own frit by crushing cullet.... how much do you need?


You could start from batch....

You can stir the glass. One way is with a chunk of potato on the end of a metal rod, plunge it to the bottom of the melt, stir it around, and then squeeze the melt.

Or you can use a strong power drill and a rod with a slight bend on the end... it'll break up clumps.

Squeeze more... bubbles WILL fine out, eventually (usually)... it's a time/heat issue. heat is never free tho.

Can you consider sintering instead of melting?

...

Anyways, I would contact someone at Corning, Kimble or Schott for more info...

[Swampy]

Try over at Craftweb (http://talk.craftweb.com/), they're pretty helpful.

[gn0me]

Considering the overall percentage of CaO you want to incorporate, have you tried sintering? Sintering a pellet of homogeneous material will give you the most uniform results, and you won't have to bring it to full melting temperature.

Also, are you adding the CaO to pure silica, or to borosilicate or soda-lime glass? I can see problems with adding it to anything but pure silica - your common glass cullet is likely soda-lime, which already has a very high percentage of calcium in it. Calcium is a modifier of the glass structure - there's only so much the glass can incorporate into its structure.

My recommendations: obtain some silicon dioxide sand and pulverize it, your CaO, and a small amount of pure sodium chloride (fining agent). Mix thoroughly, compress into a pellet, and bring to the lowest possible temperature possible to achieve a solid mass upon cooling. The addition of Boron, Magnesium, or Aluminum may help to achieve a glassy structure. MgO, CaMg(CO3)2, Al2O3, Sodium Borate or Perborate(careful!) are nice sources of these materials....

At that high of a percentage of CaO, much is likely to remain unincorporated into the glass matrix. To achieve a truly nonporus pellet you may have to coat it in silica somehow - perhaps encasing in a quartz tube or sintering with pure quartz on the outside will help.

Typical Soda-lime glass has a CaO content of around 10% already. However, they start with CaCO3 or CaMg(CO3)2 instead of CaO and the carbon tends to burn away.



Good luck! Be sure to let us know your results :D