FreeShip- Feldspar, Granular, 80-100 Mesh- (Prompt rebate on orders with 3 or more FreeShip items!)

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We have feldspar chips in some listings that are for experiments with the Japanese "Shigaraki" pottery style. It's a process that mixes "large" (~1.5 to 5.5 mm) feldspar inclusions in the clay body that melt and swell during the firing.
This is a "granular" feldspar that is a fine/medium "sand" that is not often seen available in feldspar and can be used for whatever experimental purpose you can think of (a special effect glaze?). The sand/granules are probably too small to have much of an effect including them in a clay body for Shigaraki pottery. But if you consider the scale/size of a piece of ware, it might be worth trying in some small pottery with thin walls. It could have a visible effect...(walls of 3 to 4 mm thick?).
Of course you never can know when your might need some granular feldspar! Feldspar is such an important and universal material in ceramics, both for clay bodies and glazes. For feldspar, your choices (with our shop, at least) are the relatively small 100-200 mesh particles or relatively large 2 to 5 mm particle sizes. This material gives you something in between. This particular feldspar (if I trust that that is what it truly is, and I do, now), also has a surprise. Read on....

The supplier doesn't say what kind of feldspar this is. I was surprised to see it when it came in. And a little skeptical... This feldspar is mostly uniform save for a small amount of speckles from darker impurities, but, surprise (!), 98% of the granules are transparent (especially obvious when looked at with a microscope). I expected the feldspars mineral color to be opaque, which is usually what you see when you have larger mineral specimens of feldspar. (often nearly white or lightly colored, sometimes more deeply colored). But I did some reading and found I was misinformed. Feldspar can indeed be transparent. I found that the kind called Sanidine was brought up fairly often when transparent types were mentioned. It is a high temperature potassium feldspar (K-spar) with a formula of K(AlSi3O8). "Potash spars" (potassium feldspars) are used widely in ceramics in clay bodies and in glazes.
As a further confirmation that this material is a feldspar (and a high temp K-spar) I decided to see at about what temp some of the pure material melted. Rather than starting at cone 6 and working up to cone 10 (too much time for a simple test!), I went to cone 10 to start.
It did melt (by itself) at cone 10. It probably would have started to melt at around cone 8, and if combined with some other materials as in a glaze, it would have acted as a flux and melted below that. As far as how much melted by itself? Even at cone 10 the surface of the feldspar is not a smooth melt but rather the bumpy surface of a viscous melt (although very shiny, glassy, and vitreous through and though). It was in a small crucible in a small cone 10 electric test kiln.

Here is a brief description of sanidine from { https://www.mindat.org/min-3521.html } :
"The high-temperature, fully disordered (in its Aluminum & Silicon distribution), form of K-feldspar; other potassium-dominant feldspars are microcline (fully ordered) and orthoclase (partly ordered). It forms an (alkali feldspar) series at high temperature with high albite. Petrologically, intermediates in this series from 10% up to 36% Fspar were termed anorthoclase, and anything from 36 - 100% Kspar was termed sanidine, even where Na>K. With current classifications, the name anorthoclase is discontinued and the intermediates should be called either albite or sanidine depending on the K/Na ratio.
It is common in many potassic volcanic rocks, some high level intrusive rocks and other high temperature rocks like buchites."
About Potassium Feldspars From DigitalFire:
"Generally high potash feldspars are employed in bodies and promote vitrification by forming a glassy phase that 'cements' more refractory particles together and triggers the formation of mullite from clay mineral. It is typical to see about 25% feldspar in cone 10 vitreous bodies (1300C) and 35% at cone 6 (1200C), although porcelains may have a little more. Much below 1200C feldspar will not produce a vitreous body."....
"Potassium feldspar melts at around 1200C (higher than soda feldspar) producing a more a viscous melt than does sodium feldspar. It can also produce a crystalline phase (leucite) that contributes to opacity. Thus, in industry, potash feldspars are often used more for matte glazes while soda feldspar is used for glossy glazes."...

Feldspar in general from Wikipedia { https://en.wikipedia.org/wiki/Feldspar }:
"Feldspar is a common raw material used in glassmaking, ceramics, and to some extent as a filler and extender in paint, plastics, and rubber. In glassmaking, alumina from feldspar improves product hardness, durability, and resistance to chemical corrosion. In ceramics, the alkalis in feldspar (calcium oxide, potassium oxide, and sodium oxide) act as a flux, lowering the melting temperature of a mixture. Fluxes melt at an early stage in the firing process, forming a glassy matrix that bonds the other components of the system together. In the US, about 66% of feldspar is consumed in glassmaking, including glass containers and glass fiber. Ceramics (including electrical insulators, sanitary ware, pottery, tableware, and tile) and other uses, such as fillers, accounted for the remainder.
In earth sciences and archaeology, feldspars are used for K-Ar dating, argon-argon dating, and luminescence dating. In October 2012, the Mars Curiosity rover analyzed a rock that turned out to have a high feldspar content."

Some info on Japanese "Shigaraki", mentioned above:
There isn't a lot of info about this on the internet, aside from some forum threads on Ceramic Arts Network.
There are some references to Japanese "Shigaraki" pottery (Shigaraki is a region in Japan) that can be found. Here's a link to an article on Japanese ceramics with feldspathic inclusions:
{ https://ceramicartsnetwork.org/pottery-making-illustrated/article/shigaraki-surfaces/# }
"Shigaraki: A pottery and stoneware made in the Shigaraki region of Japan. Wares from this region are known for containing large feldspar inclusions in the clay body that melt and swell during the firing process."
These "chunks" of feldspar are used as small coarse inclusions in clay bodies. They're size range is mostly 1.5 mm to 5 mm (although there's a little granular powder included and dimensions higher than 5.5 mm). Shapes are elongated or blocky, tending to be angular. Light colors dominate with the primary being white and some light pinks, green and scattered darker browns. Feldspar is the primary mineral but there are a few others, such as mica. When a ceramic piece is fired the chunks melt or semi-melt and show up as part of the ceramic body as spots of color or erupting bumps depending on the clay, feldspar, and the firing temperature. Or they can modify part of the glaze if there is one. Granite chunks are another popular inclusion.

Some extensive general info on feldspar from:
https://www.ima-na.org/page/what_is_feldspar
"The minerals of which the composition is comprised between Albite and Anorthite are known as the plagioclase feldspars, while those comprised between Albite and Orthoclase are called the alkali feldspars due to the presence of alkali metals sodium and potassium. The alkali feldspars are of particular interest in terms of industrial use of feldspars. Amongst the numerous rocks in which they are present, feldspars are particularly abundant in igneous rocks like granite, which contains up to 50% or 70% of alkaline feldspar.
Basically, the two properties which make feldspars useful for downstream industries are their alkali and alumina content. On those elements we can distinguish three families: Feldspathic sand, Pegmatite and Feldspar. A further distinction can be made between sodium, potassium and mixed feldspars, depending on the type of alkali they contain. Feldspars play an important role as fluxing agents in ceramics and glass applications, and also are used as functional fillers in the paint, plastic, rubber and adhesive industries.

--Glass: Feldspar is an important ingredient in the manufacture of glass and an important raw material as well, because it acts as a fluxing agent, reducing the melting temperature of quartz and helping to control the viscosity of glass. The alkali content in feldspar acts as flux, lowering the glass batch melting temperature and thus reducing production costs.
--Ceramics: In the manufacture of ceramics, feldspar is the second most important ingredient after clay. Feldspar does not have a strict melting point, since it melts gradually over a range of temperatures. This greatly facilitates the melting of quartz and clays and, through appropriate mixing, allows modulations of this important step of ceramic making. Feldspars are used as fluxing agents to form a glassy phase at low temperatures and as a source of alkalies and alumina in glazes. They improve the strength, toughness, and durability of the ceramic body, and cement the crystalline phase of other ingredients, softening, melting and wetting other batch constituents.
--Fillers: Feldspars also are used as fillers and extenders in applications such as paints, plastics and rubber. Beneficial properties of feldspars include good dispensability, high chemical inertness, stable pH, high resistance to abrasion, low viscosity at high filler loading, interesting refractive index and resistance to frosting. The products used in such applications are generally fine-milled grades.
--Enamel frits and glazes: Feldspar assists the enamel composition, assuring the absence of defects and the neatness of the end product: e.g. enamel frits, ceramic glazes, ceramic tile glazes, sanitaryware, tableware, electrical porcelain and giftware.
--And many other end-uses: paint, mild abrasives, urethane, welding electrodes (production of steel), latex foam, the welding of rod coating, and road aggregate.
--In the flooring sector, feldspar is the main constituent in the body composition. It is used as a flux, lowering the vitrifying temperature of a ceramic body during firing and forming a glassy phase. Surface tension pull the remaining solid particles together, giving a densification of the ceramic body. With rising temperatures the alkalis become more active and first dissolve the clay particles and then the free silica."