Frosty,

I agree with you completely...as an end goal. However, cheap hard firebricks make good "training wheels." The thing I like best about "add-ons" (everything not built into or unto the forge), is that people can always up date later. I kinda like luring folks down to the deep end, before they know it's getting serious 

Mikey The Sneak

Frosty, is this the information you wanted to dredge up?

None Stabilized zirconium dioxide (ZrO2; AKA zirconia) has three phases: Mono-clinic at less than 2138 °F (1170 °C), tetragonal between 2138 °F and 4298 °F (2370 °C). The transition between the first and second phase creates enough expansion to prevent it being used in hard refractory products, unless it is stabilized in the cubic form, or in its more useful partially stabilized tetragonal form. A small percent of calcium, yttrium, or magnesium oxides can be used to partially stabilize zirconia; cerium oxide can also be used, but is too expensive for home-built equipment. Further high temperature manipulation can form fully stabilized zirconia, but adds further expense.

    Zirconia has very low thermal conductivity, yet very high luminosity when incandescent temperatures are reached. These two facts combine to make it a preeminent heat barrier. Because of the high luminosity, it can be used as an effective method of heat transference on high temperature casting crucibles, when applied in very thin coatings (.040” or less), and yet thicker coatings can be used to “reflect” heat through re-emission, while providing insulation that only improves as heat levels rise. When it comes to various heat barrier coatings, very fine particles of zirconium are desired, because the finer the particles the higher re-emission percentages go.

    Government sponsored experiments in the nineteen-sixties showed that phosphoric acid was able to hold none-stabilized zirconia onto heating surfaces despite phase change resizing; it was an important find—back then. But stabilized zirconia is much cheaper than it was in the past, and so this more expensive product is the better choice for tough heat barriers, and nowadays for some castable refractory crucibles; clumps of it are also used as insulation between crucibles and wire windings in induction furnaces. Zirconia based refractories, and alumina ceramics with stabilized zirconia included are well known for thermal shock resistance and resistance to erosion from incandescent liquid metals.

Note: Drying can produce up to 4% shrinkage in slip cast zirconia refractories, and firing at 3452 °F (1900 °C) will produces up 15% contraction; factors to be considered when planning structures made of it. Zirconia is available for use as grog, and is an effective loose insulation for very high heat environments (think of it as being like Perlite on steroids). Zirconia also comes as stabilized ultra-high temperature porous insulating brick.

Zirconium silicate: Hobbyists concoct a tough sealant coating that is also a high-emission product bt purchasing zirconium silicate flour from a pottery supplies store, and mixing it with bentonite clay powder; this is practical, because it does not go through phase shifts.  Zirconium silicate, while very tough is only rated at about 70% heat reflection; it is also very resistant to borax, and an economical choice. Zirconium silicate can be either a coating or a hard refractory layer, depending on the amount of bentonite clay, etc. it is mixed with.

    One of the hobby blacksmiths on IFI makes a slurry of Zircopax (a brand of zirconium silicate) mixed into colloidal silica (AKA fumed silica) and a little water; he also uses this mix for shell casting; he suggests mixing it to about the consistency of latex paint, in a clear lidded jar. The Zircopax will settle out, once you stop stirring every few minutes, and cake on the bottom of the jar, with the silica and water remaining in solution over it; until it is broken up with a butter knife, and thoroughly remixed back into solution.

     When combined with silica as a binder, I believe the overall performance of Zircopax in thicker layers will prove to be considerably higher than 70% heat reflective, since the other part of its molecular structure is clear natural silicate, which will pass light rays with very little interference, and since its re-emission mechanism is radiance, I believe its overall performance in thicker layers will prove to be much higher than it is rated at. Remember that each layer must be fired before the next layer is painted on.

    Tony Hansen, of Digital Fire fame, uses Zircopax as both a coating and a solid refractory, very like clay, but good to very high temperatures, and highly insulating; two qualities that mere clay lacks. Mr. Hansen mixes it with Veegum T (a smectite clay) as a binder and plasticizer. A mixture of 97% Zircopax and 3% Veegum can be molded into structures, as easily as potters clay.  A mixture of 95% Zircopax and 5% Veegum provides a hard tough heat reflective coating for other refractory structures.

    Mr. Hansen has also used his formula to created his own 5mm thick (just over 3/16”) kiln shelf, which he states “will perform at any temperature that my test kiln can do, and far in excess of that.” It consists of 80% Zircopax Plus, with 16.5% #60 to #80 grit Molochite grog, and 3.5% Veegum T; he states that the mixture is plastic and easy to roll out, with 4.2% shrinkage, with 15.3% water added, but suggests that you dry your forms between sheets of plasterboard, to prevent warping. Firing to cone 4 produced 1% shrinkage, and left his shelf only cinder bonded.

Firing to yellow heat will produce further shrinkage, but strengthen the final product; this has about the same thermal shock resistance as high-alumina cast refractories. Avoid uneven heating by setting your forge or kiln up to work as a radiant oven.

Read about Zircopax at: https://digitalfire.com/material/zircopax  

Read about Veegum at: https://digitalfire.com/material/1672  

Right you are Sneaky, it slipped my mind. (like that's news) Now I recall recommending folks use hard fire brick to model propane forges, I have a case of hard and one of K-26 for the purpose. Hard bricks for quick models to check and refine flame flow patterns and the soft brick to build portable user forges. 

And yes again, that's what I was trying to remember about the particulars of zirconia. Thank you.

I'd say more but it looks like it's going to be another beautiful day in the northland and we're going to have a cook out for dinner. I'm off to set things up.

Happy Memorial day to all who celebrate it.

Frosty The Lucky.