US3546037A

US3546037A - Polishing crystal glass

Info

Publication number: US3546037A
Application number: US552425A
Authority: US
Inventors: Erich Salzle
Original assignee: Individual
Current assignee: Individual
Priority date: 1965-05-26
Filing date: 1966-05-24
Publication date: 1970-12-08
Anticipated expiration: 1987-12-08
Also published as: DE1496666A1; DE1496666B2; GB1110155A; DE1496666C3

Description

United States Patent 3,546,037 POLISI HNG CRYSTAL GLASS Erich Siilzle, 134a Georgenstrasse, 8 Munich 13, Germany No Drawing. Continuation-impart of application Ser. No. 322,048, Mar. 26, 1963, now Patent No. 3,290,193. This application May 24, 1966, Ser. No. 552,425 Claims priority, application Germany, May 26, 1965, S 97,315 Int. Cl. C030 15/02 US. Cl. 156-24 8 Claims ABSTRACT OF THE DISCLOSURE Glass articles are polished by being contacted with a bath containing sulfuric acid of at least 63% concentration and also containing hydrofluoric acid in which the concentration of fluorosilicic acid formed in the bath is maintained at below saturation.

This application is a continuation-in-part of Ser. No. 322,048, filed Mar. 26, 2963, now Pat. No. 3,290,193.

The present invention relates to a method of polishing glass, and more particularly to the polishing of crystal glasses.

In the known processes of polishing pressed or blown glass objects, the same are immersed for a short time in a polishing bath consisting of dilute sulfuric acid containing hydrofluoric acid, then taken out of the polishing bath and the salt layer formed on the surface of the glass is removed in a rinsing bath. This alternate treatment is repeated several times until the desired surface brilliance is achieved.

The rinsing bath in former times was water. However, this had the disadvantage that as a result of the repeated immersions a large amount of water passes into the polishingbath so that after a short time the polishing bath has to be partly regenerated. This provides an additional problem in that acid waste liquors of such concentrations, particularly those containing substantial amounts of hydrofluoric acid, may not be discharged by being passed into public streams.

To overcome these difii'culties, a process was proposed in which the rinsing bath used was concentrated sulfuric acid of such high concentration that it acts as a dehydrating agent. In this way, the salt layer on the surface of the glass resulting from the polishing bath is likewise removed and all that then remains to be done on attaining the desired brilliance after a suflicient number of washings is to rinse the completely polished object once with water. This procedure in general obviated the problems of the exhausted acid.

Another improvement in the above described process was the addition of heavy metal salts, such as iron chloride, iron sulfate, zinc sulfate, copper sulfate, silver sulfate or also potassium permanganate or chromic acid to the polishing bath for the purpose of reducing the number of alternate treatments.

All of the known processes were in general carried out using in the polishing bath a concentration of sulfuric acid which did not exceed 60%. A report has been published on scientific investigations into the properties of polishing baths with various concentrations of sulfuric acid and hydrofluoric acid, which shows that a sulfuric ICC acid concentration of about 60%, particularly a sulfuric acid concentration above 63%, gives rise to serious flaws in the glasses. It was found that when the concentration of sulfuric acid in the polishing bath reaches this high level, precipitations of fluorosilicic acid occur, and this acid forms spots or bulges on the glass which are extremely difficult to remove. These precipitations of fluorosilicic acid on the glass surfaces are due to an oversaturation of the bath, and this oversaturation prevents decomposition of the acid to silicon tetrafluoride which can no longer escape from the bath when such saturation level is reached.

It is accordingly a primary object of the present invention to provide a new method of polishing glass articles, and more particularly crystal glass articles, which avoids all of the enumerated disadvantages of the known methods and which provides for the possibility of achieving the desired brilliancy by a single immersion of the article in the polishing bath.

It is yet another object of the present invention to provide a simple and direct method of achieving a desired brilliancy on a glass article by immersion of the same in a polishing bath, using only a single immersion wherein improvements are also obtained in connection with the subsequent rinsing of the polished glass article.

Other objects and advantages of the present invention will be apparent from a further reading of the specification and of the appended claims.

With the above and other objects in view, the present invention mainly comprises a method of polishing crystal glass, which comprises contacting the glass to be polished with a bath containing sulfuric acid of at least 63% concentration and also containing hydrofluoric acid, while maintaining the concentration of fluorosilicic acid which is formed in the bath below the saturation level of said fluorosilicic acid.

Thus, the present invention provides for the polishing of crystal glass articles, especially glass containing less than 10% of lead, which includes highly leaded pressed, pre-pressed or thick walled handblown glasses in a polishing bath containing both sulfuric acid and hydrofluoric acid, wherein the sulfuric acid concentration is maintained at at least 63%, and the amount of fluorosilicic acid in the bath is kept below the saturation level.

It is a particular advantage of the present invention that it is possible to achieve the desired brilliancy by a single immersion of the article to be polished in the polishing bath, and as a result, it is not necessary to carry out repeated alternate treatments in a polishing bath followed by a water bath or a sulfuric acid bath. Consequently, the method of the present invention provides for the possibility of avoiding costly apparatus and also provides for a reduction in the number of personnel required for the transfer of the baskets loaded with the glass articles from one bath to the other.

By the immersion of the articles to be polished in the polishing bath for a period of from a few minutes up to about 20 minutes, without any alternate treatment, excellent surface quality and particularly handsome brilliancy are achieved Without formation of any waste liquids or dilutions of baths. Due to the particularly high sulfuric acid concentration which is used by the process of the present invention, the glass-covering layer formed on the glass surface by the polishing reaction is immediately dissolved and the polishing reaction may then be continued for the required time without any obstruction of a salt coating.

I have found that as soon as the saturation of fluorosilicic acid is reached, precipitations of fluorosilicic acid on the glass surface occur and this causes the marring of the articles and consequent rejection thereof. Therefore, in accordance with the present invention, the concentration of fluorosilicic acid in the polishing bath must be maintained at below the saturation level thereof by suitable means, for example by continuously withdrawing part of the liquor and replenishing the withdrawn part with fresh polishing acid.

The saturation concentration of fluorosilicic acid (HgsiFg) depends to a slight extent on the sulfuric acid concentration of the bath which, according to the present invention, is at least 63%, and is generally between about 63% and 75 In general, the permissible degree of concentration of fluorosilicic acid in the bath is from 2.5% to 4% of H SiF however, it is advantageous to maintain the range at between about 3% and 3.8%.

In practice, when the concentration of the fluorosilicic acid approaches the above mentioned level, there appear on the surface of some of the articles to be polished, small deposits, at first detached, which are not as yet harmful. Since in practice the polishing operation is generally carried out in vats of substantial capacity, and since the concentrations of the individual ingredients of the bath are not identical in all areas, there may already occur within localized zones in the bath spots of saturation before the bath as a whole exhibits an inadmissably high concentration of fluorosilicic acid. As soon as the first signs of deposition of the fluorosilicic acid occur, it is in most cases sufficient, as the first expedient, to stir the bath vigorously and then to resort to dilution of the bath for the continued performance of the polishing operation. It is not necessary to interrupt the polishing operation at the first signs of increasing concentration since, as indicated above, an undesirably high concentration of H SiF has not as yet occurred in the bath as a whole.

The concentration of fluorosilicic acid in the bath can be checked at any time :by periodically taking specimens and analyzing the same. For this purpose, a method of volumetric analysis is available which permits the determination of the amount of H SiF present in liquids containing hydrofluoric and sulfuric acid (Method of N. S. Nikolajev; note Gmelin, Silicon part B, page 196).

The concentration of hydrofluoric acid in the polishing bath of the present invention is maintained within the normal limits, namely from about 3% to 10%. Likewise,

the temperature used for the polishing is also maintained within normal limits, e.g. between about 35 C. and 70 C. To prevent an accumulation of salts resulting from the polishing reaction in the polishing bath, the salt deposits may be removed in any of the customary manners, for example by filtration, cooling and sedimentation. The removal of salt collecting on the bottom may be brought about, for example, at the same time when the bath is being replenished for the purpose of maintaining the concentration of fluorosilicic acid within the bath below the saturation level.

To maintain the desired concentration of hydrofluoric acid in the polishing bath it is advantageous to introduce the hydrofluoric acid continuously near the bottom of the bath, for example, by means of distributor pipes. Particularly good results have been obtained by introducing hydrogen fluoride in gaseous condition.

The following examples are given to further illustrate the present invention. The scope of the invention is not, however, meant to be limited to the specific details of the examples.

EXAMPLE 1 The articles to be polished are made of thick walled, heavy, hand blown lead crystal. The glass contains 24% of PhD, 59% of SiO 0.1% of BaO, 12% of K 0, 3% of Na O and 1.8% of B 0 The articles are immersed in a polishing bath consisting of sulfuric acid of 65% concentration, containing 5% of hydrofluoric acid, at C. The articles are left in the polishing bath for 10 minutes, while being occasionally moved about in it, and are then taken out and all the acid rinsed off with pure water.

The polishing effect obtained in this manner is perfect. No deposition of fluorosilicic acid appears on the glass surfaces.

EXAMPLE 2 The articles to be polished are thick-walled, pressed lead crystal articles containing 19% of PbO, 63.5% of SiO 12.5% of K 0, 3.5% of Na O and 1.5% of B 0 The procedure used in Example 1 is repeated with the exception that the sulfuric acid concentration is maintained at 72%, the hydrofluoric acid concentration at 7%, and the temperature of C. is used. The time the articles remain in the polishing bath is 8 minutes, after which the acid is rinsed off with water of 50 C. The resulting brilliance and surface quality are outstanding.

EXAMPLE 3 The articles to be polished are made from potash glass containing 3% of PhD, 73% of SiO 3.5% of BaO, 12% of K 0, 4% of Na O and 4.5% of B 0 The glass articles were previously ground with diamond discs. The polishing bath used contains 63% of sulfuric acid and 5% of hydrofluoric acid and is heated to 55 C. After a polishing time of 12 minutes, the desired degree of brilliance is achieved. The glass articles are taken out of the polishing bath and the acid rinsed off with water at 50 C.

EXAMPLE 4 The glass articles to be polished are made from thinwalled, lead-free soda glass containing 75% of SiO 4% of BaO, 4.5% of K 0, 12% of Na O and 4.5% of B 0 The glasses were first ground with natural stones.

The polishing bath contains 66% of sulfuric acid and 4% of hydrofluoric acid and is heated to 50 C. The polishing time is 8 minutes. After this time, the desired degree of brilliance is attained, the glass articles are taken out of the polishing bath, and the acid rinsed off with water at 50 C.

To renew the hydrofluoric acid in the polishing bath during the polishing operation hydrogen fluoride gas is introduced into the bath.

Although the invention has been described in particular with respect to the polishing treatment of particular glasses, it is to be understood that variations and modifications of the invention can be made without departing from the spirit or scope of the invention. Such variations and modifications are accordingly meant to be comprehended within the meaning and scope of equivalents of the appended claims.

I claim:

1. The method of polishing a glass article, which comprises contacting said glass article with an aqueous polishing bath containing 63% to about 75% by weight of sulfuric acid and also containing about 3% to about 10% by weight of hydrofluoric acid, both based on the total weight of the bath, while maintaining the concentration of formed fluorosilicic acid in the bath at below saturation level and not blowing air through the bath to carry gaseous hydrogen fluoride to the glass article, removing the glass article from the polishing bath and then washing the glass article with water.

2. Method according to claim 1 wherein the content of hydrofluoric acid in the bath is renewed by introducing hydrogen fluoride gas therein.

3. Method according to claim 1, wherein the concentration of fluorosilicic acid in the bath is maintained at between about 2.5% and 4% by weight, based on the total weight of the bath.

4. Method according to claim 1, wherein the concentration of fluorosilicic acid in the bath is maintained between about 3% and 3.8% by weight, based on the References Cited total weight of the bath. E A ENTS 5. Method according to claim 1 wherein the tempera- UNITED STAT S P T b t 2,999,013 9/1961 Meth 1566 2 ggl gf bath mamtamed between a 3,290,193 12/1966 Salzle 15614 6. Method according to claim 1 wherein the glass 5 1,777,321 10/1930 Meth is lead crystal glass.

7. Method according to claim 1, in which said contact- JACOB STEINBERG Pnmaly Examiner ing with polishing bath is performed in only a single US Cl XR stage. w'

8. Method according to claim 7, in which said washing 10 is performed in only a single stage.