This sounds a bit dangerous.  Hot, invisible exhaust gases will rise toward the torch and hoses (and gloved hand).  Can't you heat from below or maybe inside a furnace (even a 'dead' one, to hold the heat), using the torch for heat?

What material are you trying to heat, and to what temperature? Do some initial calculations to estimate what temperature can be reached for the steady state heat input of the torch.  
I have done glassblowing, and it is difficult to get a large part hot because the majority of the surface is cooling. If you don't have a furnace, maybe you can build an oven out of firebrick to hold the heat.

Good luck and take care,
Ken

What I want to heat is sand (more or less) to produce SiO2 glass direclty in the molding. The torch will produce in the welding tip about 3000C. This is more than I need and I believe it will descrease pretty much the viscosity. Maybe you know more about it. The spot with this heat is arround 1 square cm, but continuously moving the torch (not by hand) I hoped to be enough.

Could you help me with some more ideas about how should I build such an oven?

Not sure I understand.  Are you trying to glaze the surface of a sand mold (e.g., for casting liquid metal)?

Depending upon the temperature and thermal shock resistance you need, maybe you can use a lower melting glass than pure silica (m.p. ~1730^oC).

What I want is to produce very high content silica glass. The only constraint is that I cannot pure it after I melt it. It has to be produced right there within the mold. The aim is to obtain the pure silica, so that is why I'm trying to use H2-C2 flame (sorry about the mistake in the title - I've seen  only now what I wrote there is H2-C2 torch not H2-O2).

Is the only cheap way (I think) to produce the temperature and I hope also the amount of heat needed for melting it as to obtain a very low viscozity very high content of SiO2 glass before solidification occurs.

I still don’t understand exactly what and why you want to do this – seems like re-inventing the wheel – why don’t you just buy high-purity fused silica glass?
I used to buy fused silica (tubing, rod & wool) from GE (Willoughby, Ohio), Corning and Norton; here are some distributors:
http://www.quartz.com/ for tubing, rod, wool
http://www.valleydesign.com/ge124-av.htm for 100 mm diameter clear wafers
http://www.sciner.com/Opticsland/FS.htm for optical grade products and properties
http://www.ispoptics.com/product.html for optical fiber and other products
http://www.besoptics.com/html/dynasil_fused_silica.html fused silica for optics, describes different purity grades and
http://www.quartz.saint-gobain.com/downloads/Product%20List%20-%20English.pdf an international supplier of all sorts of high purity fused quartz items (crucibles, yarn, optics, etc.).

Should you still want to do your own melting, note that fused silica has an extremely low CTE (coefficient of thermal expansion), lower than that of any mold material such as graphite, so there could be a cracking problem with cast silica.

Also, if you are concerned with high purity, an H₂-O₂ flame is cleaner than oxy-acetylene.

Also note: fused silica produced from sythetic silica (from silicon chemicals) is more pure than that produced from quartz or sand.

Finally, if you are just trying to put a fused silica coating onto molds, there are flame spray methods that feed a powder into the flame which impinges onto a substrate.  This is often done with metals to build up a worn surface, or with ceramics powders such as alumina to produce a wear-resistant surface.

This is for sure not for coating. It's intended to produce very low (the lowest possible) CTE, block like, fused silica (with specific geometry).

I think the posts you made are very very usefull to me. One more thing: can you give me some tips about a small furnance for melting this SiO2 soupe? ;) What refractary materials whould you suggest? I've found some castable ceramics (zirconia, etc.) on the web, but I'm not sure they best fit my needs.

I don't believe it is worthwhile to do your own glassmaking.
Re furnaces:  If you don't have a high-temperature furnace available, I was just thinking of loosely stacking refractory bricks into a kiln shape to hold the heat.  Suggest doing a web search for glassmaking & glassmaking furnaces; I don't have any special knowledge in this area.

Graphite might work for casting molds, but it oxidizes in air at SiO₂ m.p. and probably reacts to form SiC + CO₂, so not suitable for a melting crucible (unless coated).

Try ZYP Coatings to see whether one of their high T coatings such as BN is suitable.
http://www.zypcoatings.com/

From my experience building special kilns (up to 1200 C) I would think that building a kiln starting totally from scratch would be a lot of work.  

Also the melt points given for glass and silica cover a huge range (see below)

Maybe find someone (ceramics shop, school, artist) who has a kiln and buy some  space.  

Also look into “flux”

Tom

Google
Melting point silica
Glass maximum kiln temperature

The art of glass making traces its roots to c. 3000 BC when it was used as a glaze on ceramic vessels. Over the years, the art of glassblowing was achieved in which huge furnaces of high temperature flames are used to melt the components of glass. These inexpensive raw materials are mostly silica sand (a.k.a. silica sand), soda ash (sodium carbonate) and limestone (calcium carbonate). Depending on it's composition, it can have a melting point of about 1400-1600 °C. (2900 F)  However, there are glasses that will melt at lower temperatures but those are not common among commercial nor industrial usage.

What teacher hasn't heard the age-old complaint, "I'm going to be an artist. Why do I need to know science?" Whether the medium used is film, ceramics, paint, or glass, science actually plays an important role in the outcome of the piece. Glass artists, for example, use kilns to bring glass to temperatures exceeding 1300° F. Glass has unusual characteristics at varying temperatures which the artist can utilize to produce beautiful products. The artist is confined by the laws of nature and must be aware of heating and cooling rates, as well as soaking temperatures. Violation of these laws results in shattered glass, frustration, and loss of time and money.

This is G o o g l e's cache of http://www.kilns.co.uk/gas_kilns

As has been said previously, there are many problems in producing fused silica yourself. I work in the fused quartz industry and would not recommend that you try this.

You need a furnace that can operate a 2000 degrees C, you will also need special refractories that will not contaminate the melt, you will also need to melt under vacuum conditions to avoid bubbles, etc, etc.

There are relatively few commercial manufacturers of fused quartz and fused silica products in the world (compared to manufacturers of other types of glass) because of the difficulty and knowledge needed to produce a satisfactory material!