a thixotropic , aqueous acid gel etchant composition comprises an acid , a colloidal silica sol , and an optional organic thickening agent . use of a colloidal silica sol thickening agent results in a thixotropic composition which is not stringy and which does not run upon application to the tooth surface , yet which is nonetheless liquid enough to not clog dispensers , e . g . needles , having small lumens . the preferred acid is phosphoric acid , present in an amount effective to provide etching within about 2 minutes , preferably within about 1 minute . such quantities are generally in the range from about 10 to about 50 weight percent of the total composition . other suitable acids include mineral acids such as nitric acid , hydrochloric acid , or sulfuric acid , and organic acids such as tartaric acid , maleic acid , itaconic acid , 5 - sulfosalicylic acid , propionic acid , citric acid , oxalic acid , or lactic acid . combinations comprising at least one of the foregoing acids may also be used . effective quantities of these acids generally provide a ph of around 1 , and are readily determined by one of ordinary skill in the art . colloidal silica sols consist of microfine particles of silicon dioxide dispersed in water . such colloids may be obtained by dispersing negatively charged amorphous silica particles in water . in the present composition , the sol particles are “ pearl - like ” or elongated . the particles of pearl - like colloidal silica sol preferably have diameters in the range from about 10 to about 60 nanometers . such sols are commercially available from nissan chemical , under the tradename snowtex . the diameter of the particles of the “ st - psm ” product are in the range from about 18 to about 22 nanometers , and the diameters of the particles of the “ stp - sl ” product are in the range of about 40 to about 50 nanometers . the elongated particles of colloidal silica sol preferably have diameters of about 10 nanometers and lengths in the range from about 50 to about 100 nanometers . elongated , acidic colloidal silica sols are available from nissan chemical under the tradename snowtex , product designation “ st - our ”. the silica portion of the sol comprises from about 3 to about 20 weight percent of the total etchant composition . optional organic thickening agents for inclusion in the present compositions are known , including for example carboxymethylcellulose , polyethylene oxide , gum or salts of polyacrylic acids . the etching compositions may include other components , for example fluoride , dyes such as methylene blue , or antimicrobial agents . the acid gel etchant compositions are generally formed by providing a concentrated aqueous phosphoric acid solution and adding the colloidal silica sol solution with mixing . the optional organic thickener and / or other components may be added at any time . in use , the surface to be repaired or restored is treated with the etchant composition for a period of time effective to etch the surface , and yet not damage the tooth . effective periods of time are readily determined by those of ordinary skill in the art , and are generally less than about one minute . the composition can be dispensed from a conventional push syringe or squeeze bottle , or applied with a brush . once the surface of the tooth has been treated with the acid gel etchant composition , the tooth is rinsed with water while the area is aspirated and thereafter the area is dried with a jet of air or by blotting . as with conventional acid etch compositions , care should be taken to avoid skin contact or contact with oral mucosa and eyes to prevent possible injury to the patient . conventional prophylactic , repair and restorative materials are then applied using known methods , for example sealants , primers , primer / adhesives , adhesives , glass ionomer cements , filling materials , and the like . an etching gel was prepared by mixing 50 g of st - psm solution ( about 20 % by weight colloidal silica content ) available under the snowtex product line from nissan chemical industries , ltd . with 43 g of 85 - 87 % h 3 po 4 to make a gel . seven grams of 4 % polyethylene oxide solution were added to the gel to further thicken it . the resultant gel had desirable handling characteristics and was smooth and easy to dispense through a 25 gauge tip . as a comparison , an etching gel with similar consistency as the gel in example 1 was prepared . regular fumed silica , a - 200 , available from deggusa corp . located in pa , was used as the thickening agent , and mixed with 37 % h 3 po 4 at 10 % by weight . the gel that was produced had a smooth feel , but was difficult to dispense through a 25 gauge tip . the following test shows one of the desirable properties associated with the use of colloidal silica as a thickening agent for dental conditioning compositions . heat accelerated aging tests were conducted for various acid etching compositions as indicated in table 1 below . dispensability from 25 gauge needle tips and physical appearance evaluations were performed for the etching gel compositions after exposing the gels to heat at 37 ° c . for two days in an open container . the experiments were conducted on the new gel formulation of the invention as set forth in example 1 above , regular fumed silica as set forth in comparative example 2 above , and some commercially available etching gels . film thickness and disk diameter were also measured for each etching gel “ as is ” and after heating them in an oven at 37 ° c . for two days . the film thickness and disk diameter were measured between two golgseal ™ microslides of 50 by 75 mm size using 0 . 02 g of each sample under 1 kg load for 1 minute . the test results are shown in table 2 below . for the new gel formulation of the invention containing nissan silica , example 1 above , film thickness increased ( or disk diameter decreased ) after water loss , but the fold of increase ( or decrease ) was much less than that of the gel of comparative example 2 above and for the total etch etchant . the gel of comparative example 2 contained regular fumed silica , and dried out easily . since the gel of comparative example 2 was not dispersible at a 1 kg load , its film thickness and disk diameter were not measurable . the jeneric / pentron 37 % h 3 po 4 etchant and the all etch etchant , however , lost their viscosity after heat treatment , presumably due to the degradation of polymer thickener during the heat aging process . the foregoing description illustrates preferred embodiments of the invention . however , concepts employed may , based upon such description , be employed in other embodiments without departing from the scope of the invention . accordingly , the following claims are intended to recite the invention broadly , as well as in the specific forms herein .