Will porcelain polish like glass?

you can polish a void free true ceramic to a fine reflective surface as long as you use the right progression of polishing grains that end up reducing the surface blemishes below 1/4 wave of light.

So what is a true ceramic? it must be an intimate mixture of very fine grains compressed and sintered together to a void free state.

Brick is not like this, some porcelain plates are like this.

why dothey use a glaze? It is a melted material that will freeze on the surface to give a fine reflective surface all over. To polish the object with a buffing wheel might take days,

I would prefer not having to fire the porcelain a second time for the purpose of applying a glaze.

The time and energy needed add to the cost of producing the finished product. It also introduces a potential problem should the substrate be subjected to temperature changes. I'm concerned about the possibility of the glaze crazing over time as a consequence of built up stresses.

Polishing is not a concern. I'd prefer spending the extra time with appropriate compounds to obtain the reflective surface from bare porcelain.

The tasks associated with glazing only add time and expense. I'd still have to polish the glaze anyway so I've not gained anything.

Thank you. All of you. I appreciate your support.

L. J.

Glazing is necessary to obtain a polished, shiny finish on porcelain and that is true for both dry and wet-process porcelain. A non-glazed porcelain surface is rough and, in the case of dry process, may be porous (that's the primary reason electrical grade porcelain is wet process). The glaze seals, colors, and provides a smooth, shiny surface that also strengthens the porcelain by applying a compressive stress on the surface. Fired porcelain is a hard and brittle structural material that is very strong in compression and weak in tension.

The glaze appearance can also be an indication of the soundness of the underlying porcelain. If a zone of dull glaze appears on the surface of an otherwise shiny glazed surface, it's a pretty good indication of porosity in the porcelain.

Glazing is the fast and easy way to create a smooth reflective surface.

You can also carefully polish a ceramic to a reflective surface, but that it very time consiming compared to glazing.

Hi L.J.

porcelain can be polished to a fantastic quality but there will remain some defects.

At sintering the precursor materials there is considerable shrinking and shrinking is not complete so some pores remain.

This is true for any ceramic material except rare specialties like Zerodur where optical quality is needed. Zerodur is cast as a glass and later ceramised by heat treatment.

There has been considerable work done in the early 80ies in replacing the O-ring seals in household water taps. These are now made from alumina but porcelains was tied too.

Polish and flatness is perfect to our eye (below 1 µm flat and some 10nm rms rough) but there are still the pores.

So what is your application? We had problems with the pores in making replicas in polycarbonate. We got very fine bumps on the parts.

RHABE

Ceramics are necessarily crystalline in nature. Glass on the other hand is amorpohous and so is generally referred to as a supercooled liquid. Ceramics can be polished to a great level of surface flatness depending on requirement. Reflection levels may vary from material to material, but may not reach the same levels as for glass - owing to the crystalline structure. However, surface roughness needs can be comfortably met. And a conventional porcelain is normally constituted of some amount of glass phase while the remaining is crystalline. It can be polished, provided the porosity is reduced to near zero levels during the sintering / vitrification process. In such case, glaze is not required.

However, if the porcelain is porous, a glaze coat can be applied and this can also be polished. In this case, the reflection levels can be comparable to that of glass.

This device will be polished to as fine a surface as is humanly possible prior to being placed in a vacuum deposition machine and coated with pure aluminum.

It must, therefore, be able to tolerate a high vacuum without structural failure. It must also be free from outgassing of residual vapors.

Reflectivity of the final product is of the utmost importance.

Is there a specific porcelain formula or type that I should use in order to equal the reflection level of a super-cooled liquid?

I'd enjoy not having to go through a secondary glazing operation.

Thanks

L. J.

Hi L.J.

if you need optical grade surface and reflection why not stay with proven concepts?

Porcelaine is not an optical material.

See "Yoder: Optical Materials"

Depending on the quality of the optic there nearly never is: best possible, as this will cost much more than lower quality.

So spectacle glasses are much inferior to cheap camera lenses and these inferior to good or very good camera lenses and these inferior to many commercial applications (semiconductor equipment for photolithografy) and these to some research equipment.

There are three fields necessary for specification: form error (bad point transfer function), roughness (bad stray-light, bad reflection) and scratch and dig (bad stray-light).

Hubble space telescope has a big scratch but BRO (Breault Research Organisation, Tucson) could calculate the losses to be insignificant.

Adding to this will be cleaning procedure and coating.

Many applications are done with alumina mirrors some with silicon. These are the very cheap but thermally a bit expanding materials.

Demanding optics are often done in Zerodur (near zero CTE) and quartz-glass (very small and very linear TCE) or ULE quartz-glass (lower TCE but not predictable).

So if you are able to give some specs I may have a recommendation.

Coating: bare aluminum (99.99% is good, coater often use purer material up to 6N). This will give you 92% reflectivity if the coater is excellent in vacuum quality.

If you need more you need additional anti-reflection coating that has to be tailored to the wavelength(s) of interest.

Edmund Optics has a basic introduction and some standard coatings. Theory is best found in the books from McLoed.

RHABE

RJABE wrote: "if you need optical grade surface and reflection why not stay with proven concepts? Porcelain is not an optical material."

If ceramics aren't used now in optics it's mostly because the convenience of alternate materials has given them an advantage. Those alternatives may exist for others but not for me.

If what I am attempting could be done with proven materials I'd not be here asking for alternatives.

I've worked with BK7, Borofloat, plate and Pyrex glass. I've not worked with Zerodur because of the expense. I'm trying to solve a problem that cannot be solved with glass.

All the evidence suggests that fired ceramics can be made to work if one is patient and willing to work through the necessary learning curve.

Glass does not work.

L.J.

OK,

then I would try alumina if cost is acceptable.

If you need cheaper material then try porcelain.

Or get a rough porcelain sample and send to small grinding and polishing shop.

I once visited Joe Appels in Tucson but that is 20 years now. He did remarcable work in polishing single lenses and prisms including LUPI interferometers.

There are certainly many more.

RHABE

In the mechanical seals used in water pumps, the one surface is ceramic, and polished smooth enough to shine. It is also non porous.

Remember that glass is essentially a very viscous liquid..... Ceramics can be used ito line engine cylinders, and can be made fairly non-breakable. Liquids have the advantage of reflection from a smoother surface.