These conditions are valid for manufacturing and supplies of melting-pots, rings, stirrers and auxiliary products made of refractory ceramics. The products according these conditions are used in glass industry for melting, processing and homogenizing of glass.


1. Melting-pot – it means fireproof vessel of cylindrical shape with circular or oval basis designed for melting of glass batch and scrap glass in pot furnaces. The melting pots may be classified according to chemical composition, method of making, or shape (see Annex).
1a. Ring – it means ring made of refractory, which floats on the surface of molten glass and ensures that any non-homogeneities may access into point of taking.
1b. Screw stirrer – it means screw-shaped piece made of re-fractory, which revolves and this way performs stirring of molten glass.
1c. Auxiliary products – it means other products made of refractory materials, which are used for operation of glass furnaces, e.g. shaped pieces used for the en-tire/partial sealing of glass furnace’s (so-called “cakes”); panels used for sealing of pot furnace; or shaped pieces used for partial sealing of working-hole of the furnace (so-called “screenings”), etc.).

2. Internal and external “sorts” – it means the place where the melting-pot’s wall passes to the pot’s bot-tom, namely on the “S” level (see drawing).
3. Retouching – it means the final adaptation of melting-pot’s surface as made by the producer until being rigid.
4. Crack – it means breaking of melting-pot’s surface af-fecting the wall as well as the bottom (this may be caused e.g. by an impact, or by fierce temperature change at drying, or by non-homogeneous temperature rise during tempering, or by temperature changes in the melting furnace.
5. Rupture – it means breaking of melting pot’s wall; this breaking affects the surface and proceeds into the wall up to middle of its thickness, and, in extreme case the breaking may run through the entire thickness of the wall; it may be caused by mechanical force (e.g. by an impact), or by immoderate temperature changes at drying and tempering, or during operation under working ar-rangements.
6. Drying – it means depriving the pot of moisture.

a) Large-volume drying – it means the controlled drying of the pots in large rooms, at the tem-perature of about 20°C for a duration of six months.
b) Post-drying in box-kiln – it means the controlled successive drying of the pots, whereas the tem-perature of 50°C is gradually reached after seven days, namely after previous large-volume drying for a duration of three months.

7. Tempering – it means controlled warming-out of pots us-ing tempering furnace according Annex No. 2 and 3.
8. Transfer – it means transport of pots from warming-out tempering furnace into smelting furnace.
9. Post-tempering of the pot – it means controlled burning in the pot smelting furnace, in order to obtain optimum solidification of the pot according enclosed chart, and to reach the characteristics according Annex No. 3.
10. More terminology see ČSN 70 000


11.Product characteristic – this concerns only the dried and fragile products. These are very sensitive to me-chanical impacts of all kind. The well-dried pots are hygroscopic.

a)The pots may be classified as follows:

1. According to chemical composition:
– acidic, volume of Al2O3 up to 29%
– basic, volume of Al2O3 from 32% to 40%

2. According to method of making:
– manually
– cast ceramic
– pressed/pulled

3. According to shape:
– open (circular or oval)
– enclosed.

b) The other products may be classified as follows:

1. According to chemical composition:
– same as in article 11. a) 1.
2. According to method of making:
– same as in article 11. a) 2.

3. Rings, according to shape of section:

a) classic – rectangular,
b) wedge-shaped.
c) According to the prior adjustment between the cus-tomer and supplier, the rings, screw-stirrer and auxiliary products may be delivered as burned up to 1200°C.
d) Except the mentioned types according to shape, it is possible (according to the prior agreement be-tween the customer and supplier) to produce and to deliver other special types of products.

12. Technical proprieties of the order
The following needs to be stated in the order:

a) type (name) plus size of the product
b) indication of material
c) other special data and requirements according spe-cial agreement (e.g. diverse sizes, shape modifica-tions, method of packing and shipment, etc…).


13. Quality of the pot – the pot (or another product ac-cording articles 11 and 12) must conform to monitored properties according to Annex No. 1 hereof.
14. The products, which will be directly in touch with the molten glass, must not have following defects:

a) imperfectly finished of inner surface, with pro-trusive particles of opening material (the only one exception is edge of the pot, after the height of pot was cutted-off in order to obtain smaller size, in case that there was customer re-quirements for this modification),
b) abruptions on upper edge of the pot,
c) cracks and ruptures,
d) heterogeneous inclusions neither organic nor in-organic nature.

15. The major indicator of thermal processing of the melt-ing-pot is the sintering grade of its bottom. The ab-sorptive capacity of the pot bottom’s material should not exceed 10%, on the sample taken from the 1/3 of bottom in contact with molten glass.

16.The following is not regarded as defects:

a) Roughness of the surface, which occurred after smoothing of the pot.
b) Imperfectly finished outer surface of the pot.

17.The pot must be (on the upper edge) equipped with fol-lowing data:

a) name of producer
b) size
c) type of material, plus stamp of the TK (engineering inspection)
d) registration number
e) the pots which were post-dried in box-kiln needs to be properly marked (e.g. peripheral colour stripe)
f) production date
g) name of workers who produced/retouched the pot.

18. The producer is obliged, for every pot to issue prop-erly completed registration sheet and to present this one to the customer; this registration sheet will be used for complete appreciation of the pot’s quality as well as document for complaint procedure.

The customer is obliged to return this registration sheet properly completed with evaluation of pot’s qual-ity and lifetime as well as with reached quality of molten glass.


19. The supplier will negotiate the type of package with the customer, namely in a Sales agreement.

20. During loading and transfer of the products, the tem-perature must not be lower than -4°C.

21. The shipment of the pots may be performed after the lapse of following period, which begins at the date of production:
a)the pots dried using method of “large-volume dry-ing” – after 6 (six) months, and
b)the pots dried using method of “post-drying in box-kiln” – after 3 (three) months of “large-volume drying” and 1 (one) week of “post-drying in box-kiln”.

22. Immediately before the products are loaded onto truck, the producer will verify whether the products have no cracks or ruptures (this verification will be performed so, that by means of the paintbrush, the inner and outer surface of the pot will moisten with kerosene; eventually existing crack or rupture will be then indi-cated by expressively dark colouring).


23. The transport of pots will be performed by means of covered trucks.

24. The truck will be provided with accompanying document, which contains essential information about conditions concerning safety transport of the products and serves as document for acceptance of the product by the customer; the truck driver is obliged to present this accompanying document to the customer.

25.The acceptance by the customer will be performed imme-diately after arrival of the truck. The checking of goods loaded on the truck will be performed visually (displacement, battering or scraping, breakage, rup-tures, etc…). At the same time, the kerosene test must be performed (see article 22).

26. The results of acceptance will be entered into accompanying document.

27. The accompanying documents, in which the customer will enter the record about result of delivery acceptance, will be returned by the truck driver to the producer for subsequent proceeding.


28. In case of claims, the provisions of the “Obchodní zákoník” (Commercial Code) will be used.

29. Concerning physical and chemical properties, the following will be checked for the pot materials (ČSN means the Czech state standard):
– heat resistance, according to ČSN 72 6022;
– resistance against heat distortion under load, according to ČSN 72 6015;
– absorption capacity plus specific gravity after previously performed burning-out at the temperature of 1480°C for the duration of 2 (two) hours, according to ČSN 72 5010;
– changes in length as caused by burning-out according ČSN 72 1073, after previously performed drying up to 2 (two) % (weight) and subsequently performed burning-out at the temperature of 1480°C for the duration of 2 (two) hours;
– content of Al2O3, Fe2O3, TiO2 – this will be determined by means of X-ray/fluorescent method using PHILIPS spectrometer;
– the moisture checking will be performed according to ČSN 72 1080.

30. The checking of shape and dimensions of the products will be performed by means of common measuring tools. The weight of the products will be determined using scale which accuracy must be ± 2 (two) kilograms.


29. In order to ensure maximum lifetime of the pot and other products, and regarding specific properties of such products, the supplier gives following instructions:

a) The pots and other products must be stored in dry rooms, at the minimum temperature of 20°C and maximum atmospheric humidity of 40%. The dried products need to be handled with maximum caution.
The best method of storage of pots is to use stacking stands with possibility to store several pots one above another. During the handling with “raw” pot, it is necessary to avoid any impact. In case of breach of this essential requirement, the ruptures of pot will occur when tempered in the furnace.

b) Before tempering of pots, it is necessary to per-form once more the kerosene test as described in article 22, to avoid the bringing of faulty pots into operation. The tempering or the post-tempering must not be performed with glass cullet loaded into pot (in this case the bottom of the pot is covered and a heavy corrosion of non-burned bottom will occur).

c) Tempering and post-tempering needs to be per-formed according the heat treatment chart as shown in the Annex. The course of tempering and post-tempering curves must be ascending on the whole range.
The horizontal temperature gradient (between out-ermost points in pot’s diameter) as well as ver-tical temperature gradient (between the pot’s bottom and upper edge) must not exceed 50°C. The furnace’s door must not be sealed by means of re-fractory material, until the temperature is mini-mum 250°C. The record of temperature in the tem-pering and melting furnace, by means of which the heat treatment chart for melting-pots is moni-tored, is an unavoidable document in case of pos-sible claims.

d) During transfer of pots, there must be mutual re-lation of the temperature in melting furnace and the temperature of the transferred pot. The tem-perature difference must not exceed 100°C. There must not be draught in the rooms where the trans-fer is performed.

e) When performing the post-tempering of pots in melting furnace, it is necessary to maintain the visible overpressure in the furnace, namely also at the furnace’s floor area. It is necessary to measure the progress of post-tempering, by means of an additional thermocouple, which is located between the pots at the level of about 100 mm above the furnace’s floor.

f) Before the pots may be transferred, the furnace’s floor needs to be carefully adjusted to be pla-nar. The deflection of pot’s bottom must not ex-ceed 30 mm (measured over the bottom’s diameter vertically to maximum deflection).


Related standards:

ČSN 70 0000 Základní sklářské názvosloví (Essential glass terminology)
ČSN 72 1073 Stanovení délkových změn keramických surovin sušením a pálením (Determining of changes in length for ceramic raw materials by drying and burning-out)
ČSN 72 1080 Stanovení vlhkosti a hygroskopičnosti keramických surovin (Determining of moisture and hygroscopic prop-erties of ceramic raw materials)
ČSN 72 6010 Stanovení nasákavosti, zdánlivé pórovitosti, skutečné pórovitosti, objemové hmotnosti a zdánlivé hustoty vypálených keramických hmot a výrobků (Determining of absorption capacity, apparent porosity, real porosity, specific gravity, and apparent density for burned-out ceramic materi-als and products)
ČSN 72 6015 Stanovení odolnosti proti deformaci v žáru při zatížení šamotových, dinasových a tuhových výrobků (Determining of resistance against heat distor-tion under load, for products made of fire-clay, ganister, and graphite)
ČSN 72 6022 Stanovení žáruvzdornosti keramických surovin a výrobků (Determining of heat resistance for ceramic raw materials and products)