Title of Invention

MULTILAYERED GLASS TILE AND METHOD OF ITS MANUFACTURE

Abstract

Multilayered glass tile (1) having at least one intermediate layer (3) disposed between a cover glass (2) and a carrier glass (4), especially consisting of gold leaf, which is formed in such a way that areas (6) essentially free from intermediate layers are preferably left at the edges (5) of the tile in which the cover glass (2) and the carrier glass (4) arein substance directly joined to each other, preferably by melting together.

Full Text

The present invention relates to multilayered glass tiles having at least one intermediate layer, particularly consisting of gold leaf, placed between a cover glass and a carrier glass; as well as a method of manufacturing the same. Gold platings under the protection of glass have been known for centuries and are traditionally produced by some Italian companies. By the known method, the so-called "Smailtje", there are manufactured glass-gold-glass tiles mostly of very small size, such as 2 cm x 2 cm. In this case, the cover glass usually has a thickness of only 0.3 mm and mostly consists of blown glass. The carrier glass generally has a thickness of about 4 mm. Often blue-dyed glass is used as the carrier glass. According to the conventional method, gold leaf is placed upon the carrier glass and then covered with a cover glass. The glass-gold-glass sandwich is either manufactured in a preheated condition, or it is subsequently heated up high enough to have the glass "melt onto" the gold. For allowing this process of "melting on" to be carrier out, use is made of glasses, particularly cover glasses, which are very soft. Soft not in the sense of physical hardness, but soft in the sense of having a low melting point. The low melting point of the glass is necessary because at high temperatures the gold leaf has a tendency of diffusing into the glass. If the gold diffuses into the glass it will lose its gold effect and become invisible, or it will optically disappear to such a degree that the gold effect is greatly weakened. For the production of the classical Smailtje gold-glass mosaic, therefore, low-melting glasses are used at least for the cover glass, generally having a softening point of not over 600 C. Glasses of this kind, however, by nature contain great amounts of sodium and potassium and thus are of a very poor hydrolytic class (poorer than class 3). If the glass is used outdoors., it will resist hydrolytic corrosion just for a relatively short period of time and will erode after 10 to 20 years. After the alkalis have been washed out, the cover glass will become dull and the purpose of protection by glass will no longer be achieved. It is the object of the present invention to provide an improved multilayered glass tile having an intermediate layer, especially consisting of gold leaf, which eliminates the disadvantages of conventional glass tiles. According to the invention,, this object is achieved by a multilayered glass tile having at least one intermediate layer, expecially consisting of gold leaf, which is placed between a cover glass and a carrier glass and which is formed in such a way that areas essentially free of intermediate layers are left preferably at the edges of the tile, in which the cover glass and the carrier glass are directly joined to each other, preferably by melting together. The glass tile according to the invention has the advantage of a direct fusion of the cover glass and carrier glass taking place in the areas essentially free of intermediate layers. Consequently, practically any sort of glass can be used, especially glasses having excellent hydrolytic properties, such as ordinary float glass of the hydrolytic class 3 or optical and technical glasses. In the glass tile of the invention, the areas having essentially no intermediate layers are preferably formed as a closed border region around the tile edges, in which the cover glass and carrier glass are fused together, preferably making at least short-term use of a vacuum, in such a way that the vacuum is essentially maintained in the area of the intermediate layer while the cover glass and carrier glass are pressed together at normal air pressure. According to the invention, the coherence of the glass-gold-glass tile is achieved by forming a border area around the gold leaf which is free from gold leaf and in which the cover glass is directly fused to the carrier glass such that the gold leaf is enclosed as in a glass block. In doing so,the gold leaf does not have to be bonded or melted to the glass. It is sufficient to have the gold leaf airtightly enclosed in a surrounding closed glass seal. The glass til§ of the invention is held together by the surrounding gold leaf-free border area around the gold leaf. The glass tile can be baked at temperatures of 720 to 750°C, and depending on the kind of glass and the thickness of the gold leaf lower or higher temperatures can be chosen. With such temperatures the gold leaf will not be melted into the glass, but in the border region which contains no gold leaf the cover glass and the carrier glass will be fused into a single, one-piece glass product with the gold leaf preserved in its center. The invention permits the production of a glass-gold-glass tile which is stable in itself and allows, in the first place, the use of "hard" glasses having excellent hydro-lytic properties and, in the second place, nevertheless prevents the gold leaf from optically disappearing during the melting process because it does not yet diffuse into the glass at temperatures under 750°C» In the glass tile according to the invention the surrounding border area, which is essentially free from the intermediate layer and in which the cover glass and the carrier glass are in substance directly fused to each other, preferably has a width of about 0.2 to 5 mm, and particularly about 0.5 to 2 mm. Furthermore, in the glass tile according to the invention the cover glass may have a thickness of about 0.3 to 3 mm, preferably about 0.5 to 1 mm, and the thickness of the carrier glass may be at least equal to or greater than the thickness of the cover glass and preferably be about 2 to 4 mm. The thickness of the gold leaf used for the intermediate layer is preferably about 0.25 to 0.30 mμ. In a modified embodiment of the glass tile according to the invention, the cover glass and carrier glass may be essentially of the same thickness, for example 1 to 3 mm, and preferably be produced from the same glass batch, particularly from the same glass plate, which has the advantage that cover glass and carrier glass are; automatically compatible, i.e. they do not have different coefficients of expansion, such that no compatibility test of the cover glass and carrier glass will be necessary. In this embodiment of the glass tile, the side of the carrier glass facing away from the cover glass may have one or more additional glass layers fastened to it, preferably by melting together, especially from the same glass plate as the carrier glass. In this case, the additional glass layer or layers can be provided with recesses, preferably after baking the glass tile, which may be used for anchoring the glass tile to a carrier construction. The glass tile according to the invention may include an intermediate layer consisting, at least in part, of gold, silver, platinum, tantalum, copper and/or brass in the form of foils and/or layers evaporated on the cover glass and/or the carrier glass and/or layers applied in a sputtering process and/or as a colloidal solution. The method of manufacturing glass tiles according to the invention may comprise the following steps: a) Cutting out of preferably square, rectangular, three-, five-,six-cornered or polygonal, round or oval, preferably plane carrier glasses of the desired size; b) Cutting out of gold leaf pieces which are slightly smaller than the corresponding carrier glasses, thus that when putting the gold leaf centrally upon the carrier glass there will be a surrounding border area free from gold leaf of preferably about 0.5 to 5 mm; c) Placing the cut-out gold leaf pieces on the corresponding carrier glasses, leaving uncovered the above mentioned surrounding border area; d) Cutting out of cover glasses having essentially the same measurements or being larger than the carrier glasses; e) Placing the cover glasses upon the corresponding stacks each consisting of a carrier glass and a cut of gold leaf so that three-layered stacks are formed; and f) Baking the three-layered stacks, preferably by: g) Introducing the three-layered stacks, preferabla on a tray provided with a separating agent, into a vacuum furnace having a temperature of preferably about 200 to 400°C, particularly about 350°C; h) Heating up the vacuum furnace to a temperature of about 600 C at normal atmospheric pressure; i) Evacuating the vacuum furnace, preferably to about 10 to 20 Millibar,and heating up the vacuum furnace, preferably for about 20 minutes, to a temperature above the softening temperature of the glass, preferably up to about 710°C; j) Opening the vacuum tap and increasing the air pressure in the vacuum furnace to atmospheric pressure within a few seconds; k) Increasing the temperature of the vacuum furnace at normal air pressure to a final temperature of about 720 to 750°C (depending on the type of glass) and keeping the furnace temperature constant at the final temperature for about 15 minutes and cooling the tiles, preferably by: 1) Cooling down the vacuum furnace to a temperature of about 650°C; m) Taking the tray out of the vacuum furnace and transferring it into a cooling furnace having a temperature of about 500°C; n) Keeping the furnace temperature of the cooling furnace on the transformation point of the glass (about 500 C) for a period of preferably about 18 minutes per millimeter of the tile thickness; o) Cooling down the glass tiles evenly to about 480 C during a period corresponding to preferably about 18 minutes per millimeter of the tile thickness; p) Cooling for the same period of time to about 150°C; q) Rapid cooling of the glass tiles to 100°C; and r) Removing the glass tiles from the cooling furnace. For carrying out the method according to the invention, preferably at least two furnaces are used, namely a vacuum furnace and a cooling furnace. The tray containing the multilayered glass tiles is placed into the vacuum furnace at a furnace temperature of 350°C. The furnace is then heated up to a temperature of 600°C at normal air pressure. The softening temperature of normal float glass or drawn glass is about 665°C. Therefore,the furnace is evacuated in the temperature range of 610 to 710 C. Heating up in the vacuum takes about 20 minutes. During this period, the glass tiles are receiving radiant heat only from the heating spirals because there is no air to serve as a heat conducting medium. Metal foils, especially those of gold, have a very high infrared radiation. Therefore, mainly the cover glass and the gold-less borders of the glass tile are heated during the baking process in the vacuum. The carrier glass under the gold foil (or another metal foil) is somewhat slower in heating up, all the more because of being cooled by the cold tray. However, the temperature is high enough to make the soft cover glass at the edge fuse with the carrier glass to a sufficient degree to allow no passage of air. At 710°C the ball tap of the vacuum furnace is opened and the air pressure increased to atmospheric pressure within a few seconds. During this period, the air pressure presses the cover glass upon the carrier glass and joins the glass tile together. At this point the glass tile does not have any air bubbles between the gold leaf and glass or between the glasses. Since the edges are melted or at least stuck together, no more air can get into the interior of the glass tile. Now the temperatur is increased, still at normal air pressure, to 720 to 750 C (depending on the type of glass or the desired effect) and subsequently kept constant at the final temperature (e.g. 730 C) for about 15 minutes. During this period, the cover glass and carrier glass have sufficient time to melt together at the edges. The furnace temperature is then lowered to 650 C and the tray is removed and placed into a second furnace, which serves as a cooling furnace, in order to cool it down under controlled conditions. The cooling furnace has a temperature of 500 C. In it a number of trays are collected until the furnace is filled and the cooling phase is started. In doing this, the temperature has first to be kept at the transformation temperatur of the glass (about 500°C) for some time (about 18 minutes per millimeter of the glass tile thickness). Then the glass tiles are cooled down evenly to 430 C over the same period of time (about 18 minutes per millimeter of the glass tile thickness). Subsequently, the cooling process is repeated over the same period of time down to 150°C. At 150°C and there-below the tiles can be cooled more speedily down to 100°C and then be removed from the furnace. For the vacuum baking process, a vacuum of 10 to 20 Millibar is sufficient. A more intensive vacuum does not disturb the process but is not necessary. With appropriate timing, it is possible to operate at least three vacuum furnaces with a single vacuum pump, and with two cooling furnaces (if they are big enough) any number of vacuum furnaces. In carrying out the method according to the invention of "glass fusing" with gold leaf inclusion in a vacuum, operations must take place under vacuum conditions only during the short period of time while the cover glass is joined with the carrier glass, i.e. while the glass reaches its softening temperature and shortly afterwards, generally between 650 and 710°C. This technique has a few advantages. In the first place, the baking temperature can be further decreased. Even if the glass edges do not melt together one.huridred percent but there exist at least in part so-called "cold sticking places", the glass tile in itself forms a stable body when both glass layers are pressed together by air pressure. Thus the vacuum-baked glass tile is held together not only by a surrounding melted-together border but also by air pressure. As long as the fusion at the edges of the tile seals the interior containing the gold leaf in an air-tight manner, the forces acting upon the edges are no longer decisive since air pressure compresses the tile so strongly that the fusion at the edges is no longer exposed to any tensile forces. Another advantage of the vacuum method is the fact that the edges are fused together free from air bubbles. If there are air bubbles present in the border area which form a passage from the outside to the inside of the tile, the gold leaf is no longer sufficiently "preserved" in its glass enclosure. Water will intrude in the course of the years, and with the small vibrations always acting upon the tile the gold leaf will ultimately be dissolved and be deposited somewhere in the water in the form of gold dust so that the tile will lose its visual effect. The removal of air bubbles in the border area in the case of a non-vacuum method requires the application of tricks which are complicated. For example, it is possible to let the edges sag down during the melting process so that curved borders will be developed. Curved borders are usually free from bubbles. Other, similarly complicated tricks are also possible. The vacuum method now presents a much simpler way of baking the glass with few rejects. In manufacturing the gold tile containing gold leaf, a carrier glass can be cut to an appropriate size, e.g. 50 x 50 mm, and a slightly smaller piece of gold leaf of e.g. 43 x 43 mm can be placed on the glass. A cover glass of e.g. 51 x 51 mm can be placed on top. The glass tile is normally baked on a tray provided with a separating agent. After baking the glass tile, the edges are ground off to the desired measurement so that the glass border projects over the gold leaf by no more than 0.5 to 2 mm. It is not necessary, however, to grind off the edges if no sharp edge is desired and if the glass has been cut to an exact measurement to begin with. In the following an embodiment of the invention will be described with further details, taking reference to a schematic drawing, in which: Fig. 1 shows a top view of the glass tile of the invention; and Fig. 2 shows a cross-sectional view along the line II-II in Fig. 1. The drawings show a three-layered square glass tile 1 having an intermediate layer 3 of gold leaf placed between a cover glass 2 and a carrier glass 4. The four edges 5 of the glass tile 1 are surrounded by a closed border area 6 which is free from the intermediate layer and in which the cover glass 2 and the carrier glass 4 are directly melted together. The intermediate layer of gold leaf or any other inclusion provided instead of the gold leaf remains undamaged in the glass tile which is produced by way of the vacuum method according to the invention. If a glass tile has not been baked in a vacuum and if there are air bubbles between the metal foil and the glass, the foil will at some time disappear from where it is located between air bubbles. For example, gold leaf will simply crumble due to vibrations and leave optical holes in the glass tile wherever air bubbles cause a "free-standing" gold leaf. This phenomenon does not occur with vacuum glass tiles according to the invention. Depending upon the type of glass, the gold leaf is featured with various effects. Drawn glass will contract somewhat upon being fused and will cause a dull, maybe even a "wrinkled" surface of the gold. This does not lead so much to a metallic effect but to a warm gold shade seen from all visual angles. In the case of float glass, the gold normally remains smoother and causes a stronger reflection. When baking float glass, there are also differences as to which side of the cover glass is placed on the tile (fire side or bath side). When using the non-vacuum method, there are normally air bubbles formed between the two glass plates and the gold. The majority of the air bubbles are between the cover glass and the gold, but also between the gold and the carrier glass. In the course of time, these air bubbles lead to a separation of the glass from the gold leaf. The cover glass will then, partly or entirely, lose its contact to the gold leaf, and a delicate air film will be formed between the gold and the glass. This leads to a different visual effect. Viewed from the front, the tile looks golden. Looked at from the side, beginning with a certain visual angle there will appear a silvery effect. This effect is caused by the total reflection of the light on the lower side of the cover glass. The effect is more pronounced if the tile is wet. The effect may appear disturbing, but is may also be desired. The method of manufacturing glass-gold-glass tiles according to the invention is applicable to other metals as well. Thus there can be carried out a silvering process by using silver foil instead of gold leaf. Other precious metals such as platinum may also be used. Non-precious metals such as copper or brass foil (beaten gold) may also be used. Special effects may be obtained by using oxidized or otherweise treated metal foils. However, other methods of embedding gold in the glass tile are also available. For example, the gold can be dissolved colloidally and put upon the carrier glass in the form of a fluid. Subsequently the border area has to be cleaned of the gold and the cover glass put in place. After that the tile can be baked. It is also possible to apply the gold by evaporation or in an electromagnetic field (sputter method). In these cases the use of the above described vacuum method is also of advantage. According to the invention, the baking of glass tiles is possible in a vacuum or without the use of a vacuum* Both methods according to the invention, namely baking glass tiles in a vacuum or without a vacuum have their advantages and disadvantages and their justification for different applications. The tiles baked without the vacuum show a brighter gold sheen, while glass tiles baked in the vacuum have a slightly duller luster. For indoor uses, where the glass tile is not exposed to rain and weather conditions, a glass tile baked without a vacuum can be used just as well as a vacuum-baked tile, there being no danger of water intruding into the interior of the glass through a leaky place at the edge. Therefore, it is not necessary to make sure that no air bubbles are present in the border region or in the interior of the glass When using the vacuum method, the glass tile is normally heated up to the glass softening temperature by radiant heat because of the lack of air as a conducting medium. Heating by radiant heat normally causes the cover glass to be heated up more intensely than the carrier glass. This is the case because the carrier glass is protected by the infrared radiating gold layer and therefore heats up more slowly. The unequal heating of the glass tile causes a certain "wrinkling" of the gold layer during the baking process. Without the use of vacuum the gold layer of the baked glass tiles remains much smoother than with the vacuum method. This variable effect of the gold layer may be of advantage if a smooth,, metallic appearance of the glass tiles is desired. In baking the glass tile without the use of vacuum, it is possible with certain types of glass, e.g. float glass, deliberately to cause the cover glass to be detached from the gold layer. Although in this case the glass tile is completely melted together along its edges it still has a very delicate film of air in its interior. While the gold leaf then has a general tendency to stick to the carrier glass, the cover glass gets detached in the area of the gold leaf layer because of temperature variations of the carrier glass and gold leaf combination. The cover glass is melted together with the carrier glass in the gold-less border area, but since the two glasses are not pressed together by air pressure as in the case of a vacuum tiles, it is separated from the gold leaf in the central region. The air enclosed in the glass tile is sufficient to form a very thin air film between the gold leaf and the cover glass. This thin air film in its turn creates a silvery-golden mirror effect of the gold leaf adjacent to this air film of the gold tile, which may be desirable. This mirror effect of the gold leaf adjacent to the air film is particularly striking and attractive if the cover glass of the tile is moistened with water on the outside or if the glass tile is laidi out under water, for example in a swimming pool. The optical interplay of glass tiles baked with or without the use of. a vacuum may be very attractive. For example, if both kinds of glass tiles having an intermediate layer, particularly of gold leaf, are laid out in a design, this will result in an added attraction. Baking the glass tile without the use of a vacuum is accompanied by a risk of air bubbles being formed in the border region where the cover glass is melted together with the carrier glass. Such air bubbles are dangerous for the glass tile if they interrupt the closed seal of its interior. Through an air passage formed at the edge of the glass tile moisture may later penetrate into the glass tile. This may in turn lead to separation and washing out of the gold layer, or to a growth of algae, fungi or the like. Such an opening of the interior through air bubbles should therefore be avoided at all events. The method of producing the glass tile without the use of a vacuum differs from the vacuum method essentially only in that the step of evacuating the furnace during the melting phase of the glass is left out. The cover glass and the carrier glass melt together in the contact surfaces along the edge,guaranteeing the production of a stable glass tile. In this case, too, the gold leaf is enclosed in the interior of the glass tile in an air-tight manner. However, additional measures are required to prevent the formation of air bubbles in the border region of the glass tile. Preferably, the carrier glass as well as the cover glass are cut out 20 mm larger than the gold leaf. This leads to the formation of fused edges 10 mm in width. For baking, the glass tile is additionally placed upon molds having a flat surface in the center but curving downwards at the edges. Through the effect of gravitation, the melting edges of the carrier and cover glass bend downward. The wide border regions of such a tile will then be curved downward after baking. Viewed from above, the glass tile will look like a pillow. The process of bending down the edges of the cover glass and carrier glass while melting and fusing together leads to the extrusion of any air present in the border region of the glass tile having no intermediate layer. The wide rims, however, have to be sawed off and ground off after baking in order finally to produce a flat glass tile with a narrow rim. claim: Multilayered glass tile having at least one intermediate layer provided between a cover glass and a carrier glass, especially consisting of gold leaf, the tile being formed in such a way that areas essentially free from intermediate layers are left at the edges of the tile in which the cover glass and the carrier glass are in substance directly joined to each other, preferably by melting together. The glass tile of claim 1, characterized in that the areas having essentially no intermediate layers are formed as a closed border region surrounding the tile along its edges, in which the cover glass and the carrier glass are melted together, preferably using a vacuum at least for a short period of time, in such a way that the vacuum is essentially maintained in the area of the intermediate layer when using the glass til at normal air pressure. The glass tile of claim 2, characterized in that the border regions which are essentially free from intermediate layers and in which the cover glass and the carrier glass are in substance directly melted together have a width of about 0.2 to 5 mm, preferably about 0.5 to 2 mm. 4. Glass tile according to one or more of claims 1 to 3, characterized in that the cover glass has a thickness of about 0.3 to 3 mm, preferably about 0.5 to 1 mm, and that the thickness of the carrier glass is at least equal to or greater than the thickness of the cover glass and is preferably about 2 to 4 mm. 5. Glass tile according to one or more of claims 1 to 4, characterized in that the thickness of the gold leaf used for the intermediate layer is about 0.25 to 0.30 my.. 6. Glass tile according to claim 4 or 5, characterized in that the cover glass and the carrier glass are essentially of the same thickness and preferably made from the same batch of glass, especially from the same glass plate. 7. Glass tile according to claim 6, characterized in that on the side of the carrier glass facing away from the cover glass there are attached one or more additional glass layers, preferably by melting together, which preferably come from the same batch of glass and especially from the same glass plate as the carrier glass. 8. Glass tile according to claim 7, characterized in that the additional glass layer or layers, respectively, are provided, preferably after baking the glass tile, which recesses used preferably for anchoring the glass tile on a carrier construction. 9. Glass tile according to one or more of claims 1 to 8, characterized in that the intermediate layer consists at least in part of gold, silver, platinum, tantalum, copper and/or brass in the form of foils and/or of colloidal layers vaporized upon the cover glass and/or the carrier glass and/or by use of the sputter method and/or layers applied in the form of a colloidal solution. 10. Method of producing glass tiles according to one or more of claims 1 to 9, characterized by the following steps: a) Cutting out of preferably square, rectangular, three-, five-, six-cornered or polygonal, round or oval, preferably plane carrier glasses of the desired size; b) Cutting out of gold leaf pieces which are slightly smaller than the corresponding carrier glasses, thus that when putting the gold leaf centrally upon the carrier glass there will be a surrounding border area free from gold leaf of preferably about 0.5 to 5 mm; c) Placing the cut-out gold leaf pieces on the corresponding carrier glasses, leaving uncovered the above mentioned surrounding border area; d) Cutting out of cover glasses having essentially the same measurements or being larger than the carrier glasses; e) Placing the cover glasses upon the corresponding stacks each consisting of a carrier glass and a cutout piece of gold leaf so that three-layered stacks are formed; and f) Baking the three-layered stacks, preferably by: g) Introducing the above mentioned three-layered stacks, preferably on a tray provided with a separating agent, into a baking furnace, preferably in the form of a vacuum furnace having a temperature of preferably about 200 to 400°C, particularly about 350°C. h) Heating up the vacuum furnace to a temperature of about 600 C at normal atmospheric pressure; i) Evacuating the vacuum furnace, preferably to about 10 to 20 Millibar and heating up the vacuum furnace, preferably for about 20 minutes, to a temperature above the softening temperature of the glass, preferably up to about 710°C; j) Opening the vacuum tap and increasing the air pressure in the vacuum furnace to atmospheric pressure within a few seconds; k) Increasing the temperature of the vacuum furnace at normal air pressure to a final temperature of about 720 to 750°C (depending on the type of glass) and keeping the furnace temperature constant at the final temperature for about 15 minutes and cooling the tiles, preferably by: 1) Cooling the vacuum furnace to a furnace temperature of about 650°C; m) Taking the tray out of the vacuum furnace and transferring it into a cooling furnace having a temperature of about 500°C; n) Keeping the furnace temperature of the cooling furnace on the transformation temperature of the glass (about 500°C) for a period of preferably about 18 minutes per millimeter of the thickness of the glass tile; o) Cooling the glass tiles evenly to about 480°C during a period corresponding to preferably about 18 minutes per millimeter of the glass tile thickness; p) Cooling for the same period of time to about 150°C; q) Rapid cooling of the glass tiles to 100°C; and r) Removing the glass tiles from the cooling furnace. Method according to claim 10, characterized in that after the baking process the borders or edges, respectively, of the glass tiles are ground off to such an extent that the surrounding gold leaf-less border area has a width of about 0.5 to 2 mm. 12. Multilayered glass tile substantially as herein described with reference to the accompanying drawings. 13 Method of producing glass tile substantially as herein described with reference to the accompanying drawings.

Documents:

1551-mas-1998-abstract.pdf

1551-mas-1998-claims duplicate.pdf

1551-mas-1998-claims original.pdf

1551-mas-1998-correspondence others.pdf

1551-mas-1998-correspondence po.pdf

1551-mas-1998-description complete duplicate.pdf

1551-mas-1998-description complete original.pdf

1551-mas-1998-drawings.pdf

1551-mas-1998-form 1.pdf

1551-mas-1998-form 26.pdf

1551-mas-1998-other documents.pdf

abs-1551-mas-1998.jpg