Refractory coatings 101

[Mikey98118]

 

Sealing and high-missive coatings for ceramic fibers and other refractory surfaces

Even rigidized ceramic fiber products still need to be sealed for safety. Furthermore, the various coatings used for sealing tend to create a tough surface layer that holds high-emissive coatings from peeling away from the fiber’s surface; an irritating tendency that results from spreading high-emissive coatings directly on fiber products (especially those that aren’t rigidized first). Just as not all sealants are rated as high-emissive, not all high-emissive coatings are effective sealants, so you need to review the better-known products. There are also products, such as shell coatings that could do very well for these purposes.

ITC-100 is strictly a high-emissive coating (not for ealing); Twenty-two years ago, I found that deliberately separating it by adding more water to a small amount in a water glass, caused the non-colloidal particles to separate out, refining the coating, and greatly increasing its emission of radiant energy. My forge went from orange to lemon-yellow incandescence, with just this change.

I’m not sure ITC is the same formula today. But. you can make a better formula, for less money than this product now costs. 100% colloidal zirconium flour can be purchased from various online sources, and mixed with phosphoric acid to make a high-emissive coating, rated above 90% “reflective” of radiant heat.

 

Zirconium silicate: Some hobbyists concoct a tough sealant coating that is also a high-emissive product; they purchase zirconium silicate flour from a pottery supplies store, and mix it down with clay powder.  Zirconium silicate, while very tough is only rated at about 70% heat reflection, but I think this figure is misleading; since the other part of its structure is clear natural silicate, which will pass light rays with very little interference, and since its operating mechanism is re-radiance, I believe its overall performance in thicker layers will prove to be considerably higher than 70%; it is also very resistant to borax, and an economical choice.

    Others use a slurry of Zircopax (a brand of zirconium silicate) mixed into to colloidal silica and a little water; the same mix is used for shell casting; mix 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.

    While Plistex can be either a coating or a high alumina cast refractory, depending on the amount of water used, Zirconium silicate can be either a coating or or hard refractory layer, depending on the amount of bentonite clay, etc. it is mixed with.

[Mikey98118]

Sealing and high-emissive coatings for ceramic fibers and other refractory surfaces

Rigidized ceramic fiber products still need to be sealed for safety. Furthermore, the various coatings used for sealing tend to create a tough surface layer that holds high-emissive coatings from peeling away from the fiber’s surface; an irritating tendency that results from spreading high-emissive coatings directly on fiber products (those that aren’t rigidized first). Just as not all sealants are rated as high-emissive, not all high-emissive coatings are effective sealants, so you need to review the better-known products. There are also products, such as shell coatings that could do very well for these purposes.

ITC-100 is strictly a high-emissive coating; Twenty-two years ago, I found that deliberately separating it by adding more water to a small amount in a water glass, caused the non-colloidal particles to separate out, refining the coating, and greatly increasing its emission of radiant energy. My forge went from orange to lemon-yellow incandescence, with just this change.

I’m not sure ITC is the same formula today. But. you can make a better formula yourself. 100% zirconia flour can be purchased from online sources, and mixed with phosphoric acid to make a high-emissive coating, rated above 90% “reflective” of radiant heat.

Un-stabilized zirconium dioxide (ZrO2; AKA zirconia) has three phases: Monoclinic at less than 2138 °F (1170 °C), tetragonal between 2138 °F and 4298 °F (2370 °C),  and cubic above 4298 °F. 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 this purpose. 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 is 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 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.   When used as a refractory, it contains very large particles as grog. Zirconia based refractories, and alumina ceramics with stabilized zirconia included are famous 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% further 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 like Perlite of steroids). Zirconia also comes as stabilized ultra-high temperature porous insulating brick.

Zirconium silicate: Some hobbyists concoct a tough sealant coating that is also a high-emissive product; they purchase zirconium silicate flour from a pottery supplies store, and mix it down with clay powder.  Zirconium silicate, while very tough is only rated at about 70% heat reflection, but I think this figure is misleading; since the other part of its structure is clear natural silicate, which will pass light rays with very little interference, and since its operating mechanism is re-radiance, I believe its overall performance in thicker layers will prove to be considerably higher than 70%; it is also very resistant to borax, and an economical choice.

    One of the guys on IFI uses a slurry of Zircopax (a brand of zirconium silicate) mixed into to colloidal silica and a little water; similar mix is used in shell casting; he says to mix 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.

Plistix 900 is rated at 70% heat reflection, and makes a tough smooth sealing coat rated for use at 3400°F; This product can also be used as cast refractory, and as mortar.

Matrikote 90 AC Ceramic Coating (one of the product line from Allied Minerals) is a very tough hard fine grained high alumina refractory coating containing 90.4% alumina, 1.5 silicon dioxide as a vitreous(glass-like) binder, and 2.7 % phosphorus oxide as a polymerizing binder. Matrikote is good to 3000 °F, and would prove useful as an inner layer between outer coatings of higher use temperatures and rigidized ceramic fiber products;

Satanite is probably the best-known refractory mortar that is also used as a hard coating/sealant over ceramic fiber board; it is use rated at 3200 F, and is easily purchased in small quantities through knife making suppliers. But, mortars are not recommended as flame faces, so plan on using a different finish coating over it for interior surfaces; It is excellent on exterior surfaces.

I hope others will post accounts of their own formulas and the products they've run across here.

[Frosty]

You're off to an epic start Mike.  I'm bookmarking this as a reference and will be linking folk directly when this stuff comes up. 

Do you recommend, pure or a dilution of phosphoric acid when mixing with zirconium flour at what ratio? What dilution if appropriate? 

Frosty The Lucky.

[Mikey98118]

You can use it at grocery store strength without worry. When heated, the water goes by-by anyway. So, you could say that the polymer coating it forms is kinda self-regulating

[Mikey98118]

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 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 ither refractory structures.

 

    Mr. Hansen has also 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, when 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 minor shrinkage, but strengthen the final product; this as about the same thermal shock resistance to high-alumina cast refractories. Avoid uneven heating.

Read about Zircopax at:

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

[Mikey98118]

 Read about Zircopax at:

I don't know why the address keeps getting wiped out 

[Mikey98118]

BXI Ceramic Fiber Thermal Insulation Board is use rated to 2732F. These boards actually measure 11.81" x 7.87" x 0.39"; $20 per board through Amazon.

Perlite is steam expanded volcanic glass, which is use rated to 1,650 °F; it is very good to use as a low priced lightweight very efficient tertiary insulator, outside of ceramic wool blanket.

[Mikey98118]

Sodium silicate (water-glass) melts at 1,990 °F. The dry flakes are fairly inexpensive, and when mixed with water it becomes an excellent high temperature glue, to bond Perlite chinks together with, forming tertiary insulting solid structures in convenient shapes.

[ThomasPowers]

We have a perlite mine in Socorro NM.  Sticks together well.  I have thought of trying a massive carved monolithic version...till it cracks.