I am not a great expert in the art of making copper red glazes, but I have learned something in my 10 years of playing with them, and will share this with you here. The following are factors that generally improve the chances of getting a copper red glaze:

1) Have a high silicon content in the glaze.

2) The silica in the glaze should be very fine so that it may enter the glaze melt sooner. This may be achieved by grinding the silica and feldspar. Alternatively FFF feldspar may be used. In wood ashes the silica is tiny.

3) A mix of both sodium and potassium.

4) Have a low magnesium content, as magnesium tends to turn the reds to pinks.

5) Have a low clay content. If clay must be used, use ball clay rather than china clay.

6) Help stabalize the reduced copper by including 2 % tin oxide and 1-2 % iron oxide. During oxidation at the later end of the firing this will preferentially attract the oxygen. Some potters use reduced black iron oxide, but I see little difference to using any iron oxide since all will by oxidised by the time the kiln gets to 1000C.

7) Start the reduction early. Keep the time of the firing above 1000C to a minimum to lower the chances of all the copper volatising.

8) The copper red glazes should be in sheltered parts to minimise the loss of copper. Ideally these glazes should be in saggers that are themselves coated with copper oxide.