Comment convertir CLOB en BLOB dans Oracle?

D'abord j'ai converti une boule d'une image en CLOB, puis j'ai reconverti cette CLOB en BLOB. Je ne vois pas l'image reconvertie. Comment puis-je résoudre ce problème?

fonction blob_to_clob:

CREATE OR REPLACE FUNCTION blob_to_clob (blob_in IN BLOB) RETURN CLOB AS
  v_clob CLOB;
  v_varchar VARCHAR2(32767);
  v_start PLS_INTEGER := 1;
  v_buffer PLS_INTEGER := 32767;
BEGIN
  DBMS_LOB.CREATETEMPORARY(v_clob, TRUE);
  FOR i IN 1..CEIL(DBMS_LOB.GETLENGTH(blob_in) / v_buffer)
  LOOP
    v_varchar := UTL_RAW.CAST_TO_VARCHAR2(DBMS_LOB.SUBSTR(blob_in, v_buffer, v_start));
    DBMS_LOB.WRITEAPPEND(v_clob, LENGTH(v_varchar), v_varchar);
    v_start := v_start + v_buffer;
  END LOOP;
  RETURN v_clob;
END blob_to_clob;

fonction clob_to_blob:

CREATE OR REPLACE FUNCTION clob_to_blob2(p_clob IN CLOB) RETURN BLOB IS
  v_blob BLOB;
  v_offset NUMBER DEFAULT 1;
  v_amount NUMBER DEFAULT 4096;
  v_offsetwrite NUMBER DEFAULT 1;
  v_amountwrite NUMBER;
  v_buffer VARCHAR2(4096 CHAR);
BEGIN dbms_lob.createtemporary(v_blob, TRUE);
  Begin
    LOOP
      dbms_lob.READ (lob_loc => p_clob,
        amount  => v_amount,
        offset  => v_offset,
        buffer  => v_buffer);

      v_amountwrite := utl_raw.length (r => utl_raw.cast_to_raw(c => v_buffer));

      dbms_lob.WRITE (lob_loc => v_blob,
        amount  => v_amountwrite,
        offset  => v_offsetwrite,
        buffer  => utl_raw.cast_to_raw(v_buffer));

      v_offsetwrite := v_offsetwrite + v_amountwrite;

      v_offset := v_offset + v_amount;
      v_amount := 4096;
    END LOOP;
  EXCEPTION
    WHEN no_data_found THEN
    NULL;
  End;
  RETURN v_blob;
END clob_to_blob2;